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'5  CENTS|Nov.  1882] 


Six  LECTURES 


ON  LIGHT 


BY 


JOHN    TYNDALL 


ILLUSTRATED 


NEW  YORK 

THEHUnBOLDTPUBLI5HiN(iCOnPANY 


•NTBRKD  AT  THB  NEW  YORK  POST  OFFICE  AS  SECOND  CLASS  MATTER. 


tolwo 

lOO.OOO  SOLD. 


Its     History     and     Present     Development. 
By  FREDRIK  BJORNSTROM,  M.  D., 

Head  Physician  of  the  Stockholm  Hospital,  Professor  of  Psychiatry,  Late  Royal  Swed- 

ish Medical  Counselor. 
Authorized  Translation  from  the  Second  Swedish  Edition. 

BY  BARON  NILES  POSEE,  M.  G., 

Director  of  the  Boston  School  of  Gymnastics. 

Paper  Cover  (No.  113  of  The  Humboldt  Library),       -  30  Cents 

Cloth,  Extra,        "  "  "  -  75  Cents 


PRESS  NOTICES. 

The  learned  Swedish  physician,  Bjornstrom. — Churchman. 

It  is  a  strange  and  mysterious  subject,  this  hypnotism. —  The  Sun. 

Perhaps  as  concise  as  any  work  we  have. — S.  California  Practitioner. 

We  have  found  this  book  exceedingly  interesting. — California  Homapath. 

A  concise,  thorough,  and  scientific  examination  of  a  little-understood  subject. — Episco- 
pal Recorder. 

Few  of  the  new  books  have  more  interest  for  scientist  and  layman  alike. — Sunday 
Times  (Boston). 

The  study  of  hypnotism  is  in  fashion  again.  It  is  a  fascinating  and  dangerous  study. — 
Toledo  Bee. 

It  is  well  .written,  being  concise,  which  is  a  difficult  point  to  master  in  all  translations.— 
Medical  Bulletin  (Philadelphia). 

The  subject  will  be  fascinating  to  many,  and  it  receives  a  cautious  yet  sympathetic 
treatment  in  this  book. — Evangelist. 

One  of  the  most  timely  works  of  the  hour.  No  physician  who  would  keep  up  with  the 
times  can  afford  to  be  without  this  work. — Quarterly  Journal  of  Inebriety. 

Its  aim  has  been  to  give  all  the  information  that  may  be  said  under  the  present  state  of 
our  knowledge.  Every  physician  should  read  this  volume. — American  Medical  Journal 
(St.  Louis). 

It  is  a  contribution  of  decided  value  to  a  much-disussed  and  but  little-analyzed  subject 
by  an  eminent  Swedish  alienist  known  to  American  students  of  European  psychiatry. — 
Medical  Standard  (Chicago). 

This  is  a  highly  interesting  and  instructive  book.  Hypnotism  is  on  the  onward  march 
to  the  front  as  a  scientific  subject  for  serious  thought  and  investigation. —  The  Medical  Free 
Press  (Indianapolis). 

Many  of  the  mysteries  of  mesmerism,  and  all  that  class  of  manifestation,  are  here 
treated  at  length,  and  explained  as  far  as  they  can  be  with  our  present  knowledge  of  psy- 
chology.— New  York  Journal  of  Commerce. 

The  marvels  of  hypnotic  phenomena  increase  with  investigation.  Dr.  Bjornstrom,  in 
this  clear  and  well-written  essay,  has  given  about  all  that  modern  science  has  been  able  to 
develop  of  these  phenomena. — Medical  Visitor  (Chicago). 

It  has  become  a  matter  of  scientific  research,  and  engages  the  attention  of  some  of  the 
foremost  men  of  the  day,  like  Charcot,  of  Paris.  It  is  interesting  reading,  outside  of  any 
usefulness,  and  may  take  the  place  of  a  novel  on  the  office  table. — Eclectic  Medical  Jour- 
nal(Cincinnati). 

This  interesting  book  contains  a  scholarly  account  of  the  history,  development,  and 
scientific  aspect  of  hypnotism.  As  a  whole,  the  book  is  of  great  interest  and  very  instruc- 
tive. It  is  worthy  of  careful  perusal  by  all  physicians,  and  contains  nothing  unfit  to  be  read 
by  the  laity. — Medical  and  Surgical  Reporter  (Philadelphia). 

To  define  the  real  nature  of  hypnotism  is  as  difficult  as  to  explain  the  philosophy  of  toxic 
or  therapeutic  action  of  medicine — more  so,  indeed.  None  the  less,  however,  does  it  be- 
hoove the  practitioner  to  understand  what  it  does,  even  if  he  cannot  tell  just  what  it  is,  or 
how  it  operates.  Dr.  Bjornstrom's  book  aims  to  give  a  general  review  of  the  entire  subject. 
— Medical  Record. 


THE    HUHBOLDT   PUBLISHING  CO., 

64  Fifth  Avenue,  New  York. 


T 


S\I  LECTURES  ON  LIGHT. 


BY  Prof.  JOHN  TYNDALL,  F.R.S. 


PREFACE. 


MY  eminent  friend,  Prof.  Joseph  Henry, 
of  Washington,  did  me  the  honor  of  taking 
these  lectures  under  his  personal  direction, 
and  of  arranging  the  times  and  places  at 
which  they  were  to  be  delivered. 

Deeming  that  my  home-duties  could  not, 
with  propriety,  be  suspended  for  a  longer 
period,  I  did  not,  at  the  outset,  expect  to  be 
able  to  prolong  my  visit  to  the  United 
States  beyond  the  end  of  1872. 

Thus  limited  as  to  time,  Prof.  Henry  began 
in  the  North,  and,  proceeding  southwards, 
arranged  *for  the  successive  delivery  of  the 
lectures  in  Boston,  New  York,  Philadelphia, 
Baltimore,  and  Washington. 

By  this  arrangement,  which  circumstances 
at  l  he  time  rendered  unavoidable,  the  lec- 
tures in  New  York  were  rendered  coincident 
with  the  period  of  the  presidential  election. 
This  was  deemed  unsatisfactory,  and  when 
dhe  fact  was  represented  to  me  I  -U  once  of- 
fered to  extend  the  time  of  my  visit  so  as  to 
make  the  lectures  in  New  York  succeed 
those  in  Washington.  The  proposition  was 
cordially  accepted  by  my  friends. 

To  me  personally  this  modified  arrange- 
ment has  proved  in  the  highest  degree  satis- 
factory. It  gave  me  a  much-needed  holiday 
at  N iagara  Falls  ;  it,  moreover,  rendered  the 
successive  stages  of  my  work  a  kind  oigro?ut/iy 
which  reached  its  most  impressive  develop 
in  New  York  and  Brooklyn. 


In  every  city  that  I  have  visited,  my  recep- 
tion has  been  that  of  a  friend ;  and,  now 
that  my  visit  has  become  virtually  a  thing  of 
the  past,  I  can  look  back  upon  it  with  unqual- 
ified pleasure.  It  is  a  memory  without  a 
stain — an  experience  of  deep  and  genuine 
kindness  on  the  part  of  the  American  people 
never,  on  my  part,  to  be  forgotten. 

This  relates  to  what  may  be  called  thefos- 
itive  side  of  my  visit — to  the  circumstances 
attending  the  work  actually  done.  My  only 
drawback  relates  to  work  undone;  for  I  carry 
home  with  me  the  consciousness  of  having 
been  unable  to  respond  to  the  invitations  of 
the  great  cities  of  the  West ;  thus,  I  fear, 
causing,  in  many  cases,  disappointment. 
Would  that  this  could  have  been  averted  ! 
But  the  character  of  the  lectures,  and  the 
weight  of  instrumental  appliances  which  they 
involved,  entailed  loss  of  time  and  heavy 
labor.  The  need  of  rest  alone  would  be  a 
sufficient  admonition  to  me  to  pause  here ; 
but,  besides  this,  each  successive  mail  from 
London  brings  me  intelligence  of  work  sus- 
pended and  duties  postponed  through  my 
absence.  These  are  the  considerations 
which  prevent  me  from  responding,  with  a 
warmth  commensurate  with  their  own,  to 
the  wishes  of  my  friends  in  the  West. 

On  quitting  England  I  had  no  intention 
of  publishing  these  lectures,  and,  except  a 
fragment  or  two,  not  a  line  of  them  was  written 


559 


SIX  LECTURES  ON  LIGHT. 


when  I  reached  this  city.  They  have  been 
begun,  continued,  and  ended  in  New  York, 
and  bear  only  too  evident  marks  of  the  rapid- 
ity of  their  production.  I  thought  it,  how- 
ever, due,  both  to  those  who  heard  them  with 
such  marked  attention,  and  to  those  who  wish- 
ed to  hear  them,  but  were  unable  to  do  so,  to 
leave  tnem  behind  me  in  an  authentic  form. 
The  execution  of  this  work  has  cut  me  off 
from  many  social  pleasures  ;  it  has  also  pre- 
vented me  from  making  myself  acquainted  with 
institutions  in  the  working  of  which  I  feel  a 
deep  interest.  But  human  power  is  finite, 
and  mine  has  been  expended  in  the  way  which 
1  deemed  most  agreeable,  not  to  my  more 
intimate  friends,  but  to  the  people  of  the 
United  States. 

In  the  opening  lecture  are  mentioned  the 
names  of  gentlemen  to  whom  I  am  under 
lasting  obligations  for  their  friendly  and  often 
laborious  aid.  The  list  might  readily  be  ex- 
tended, for  in  every  city  I  have  visited  willing 


helpers  were  at  hand.  I  must  not,  however, 
omit  the  name  of  Mr.  Rhecs,  Professor 
Henry's  private  secretary,  who,  not  only  in 
Washington,  but  in  Boston,  gave  me  most 
important  assistance.  To  the  trustees  of  the 
Cooper  Institute  my  acknowledgments  are 
due ;  also  to  the  directors  of  the  Mercantile 
Library  at  Brooklyn.  I  would  add  to  these  a 
brief  but  grateful  reference  to  my  high-minded 
friend  and  kinsman,  General  Hector  Tyn- 
dale,  for  his  long-continued  care  of  me,  and  for 
the  thoughtful  tenderness  by  which  he  and  his 
family  softened,  both  to  me  and  to  the  parents 
of  the  youth,  the  pain  occasioned  by  the  death 
of  my  junior  assistant  in  Philadelphia. 

Finally,  I  have  to  mention  with  warm  com- 
mendation the  integrity,  ability,  and  devo- 
tion, with  which,  from  first  to  last,  I  have 
been  aided  by  my  principal  assistant,  Mr. 
John  Cottrell. 

NEW  YORK,  February,  1873. 


LECTURE  I. 

INTRODUCTORY  :  Uses  of  Experiment :  Early  Scien- 
tific Notions :  Sciences  of  Observation :  Knowl- 
edge of  the  Ancients  Regarding  Light:  Nature 
judged  from  Theory  defective:  Detects  of  the 
Eye:  Our  Instruments:  Rectilineal  Propagation 
of  Light :  Law  of  Incidence  and  Reflection : 
Sterility  of  the  Middle  Ages:  Retraction:  Dis- 
covery of  Snell :  Descartes  and  the  Rainbow : 
Newton's  Experiments  on  the  Composition  of 
Solar  Light :  His  Mistake  as  regards  Achroma- 
tism :  Synthesis  of  White  Light :  Yellow  and 
Blue  Lights  proved  to  produce  White  by  their 
Mixture  :  Colors  of  Natural  Bodies  :  Absorption  : 
Mixture  of  Pigments  contrasted  with  Mixture  of 
Lights. 

SOME  twelve  years  ago  I  published,  in 
England,  a  little  book  entitled  the  "  Glaciers 
of  the  Alps,"  and,  a  couple  of  years  subse- 
quently, a  second  volume,  entitled  "  Heat  as 
a  Mode  of  Motion."  These  volumes  were 
followed  by  others,  written  with  equal  plain- 
ness, and  with  a  similar  aim,  that  aim  being 
to  develop  and  deepen  sympathy  between 
science  and  the  world  outside  of  science.  I 
agreed  with  thoughtful  men*  who  deemed 
it.  good  for  neither  world  to  be  isolated  from 
the  other,  or  unsympathetic  towards  the 
other,  and,  to  lessen  this  isolation,  at  least  in 
one  department  of  science,  I  swerved  aside 
from  those  original  researches  which  had  pre- 
viously been  the  pursuit  and  pleasure  of  my 
life. 

These  books  were,  for  the  most  part,  re- 
published  by  the  Messrs.  Appleton,  under 


the  auspices  of  a  man  who  is  untiring  in  his 
efforts  to  diffuse  sound  scientific  knowledge 
among  the  people  of  this  country;  who^e 
energy,  ability,  and  single-mindedness,  in  the 
prosecution  of  an  arduous  task,  have  won  for 
him  the  sympathy  and  support  of  many  of  us 
in  "the  old  country."  I  allude  to  Professor 
Youmans,  of  this  city.  Quite  as  rapidly  as 
in  England,  the  aim  of  these  works  was  un- 
derstood and  appreciated  in  the  United 
States,  and  they  brought  me  from  this  side 
of  the  Atlantic  innumerable  evidences  of 
good-will.  Year  after  year,  invitations 
reached  me  *  to  visit  America,  and  last  year 
I  was  honored  with  a  request  so  CQrdial,  and 
signed  by  five-and-twenty  names  so  distin- 
guished in  science,  in  literature,  and  in  ad- 
ministrative position,  that  I  at  once  resolved 
to  respond  to  it  by  braving,  not  only  the  dis- 
quieting oscillations  of  the  Atlantic,  but  the 
far  more  disquieting  ordeal  of  appearing  in 
person  before  the  people  of  the  United 
States. 

This  request,  conveyed  to  me  by  my  ac- 
complished friend,  Professor  Lesley,  of  Phil- 
adelphia, and  preceded  by  a  letter  of  the 
same  purport  from  your  scientific  Nestor, 
Professor  Joseph  Henry,  of  Washington,  de- 
sired that  I  would  lecture  in  some  of  the 
principal  cities  of  the  Union.  This  I  agreed 
to  do,  though  much  in  the  dark  as  to  what 
form  such  lectures  ought  to  to  take.  In 


*  Among  whom  may  be  mentioned,  specially,  the        *  One  of  the  earliest  came  from  Mr.  John  Amory 
late  Sir  Edmund  Head,  Bart.  Lowell,  of  Boston. 


SIX  LECTURES  ON  LIGHT 


answer  to  nv  inquiries,  however,  I  was  given 
to  understand  (by  Professor  Youmans  princi- 
pally) that  a  course  of  experimental  lectures 
would  materially  promote  scientific  education 
in  this  country,  and  I  at  once  resolved  to 
meet  this  desire,  as  far  as  my  time  allowed. 

Experiments  have  two  uses— a  use  in  dis- 
covery, and  a  use  in  tuition.  They  are  the 
investigator's  language  addressed  to  Nature, 
to  which  she  sends  intelligible  replies.  These 
replies,  however,  are,  for  the  rrost  part,  at 
first  too  feeble  for  the  public  ear  ;  for  the  in- 
vestigator cares  little  for  the  loudness  of  Na- 
ture's voice  if  he  can  only  unravel  its  meaning. 
But  after  the  discoverer  comes  the  teacher, 
whose  function  it  is  eo  to  exalt  and  modify  the 
resu  ts  of  the  discoverer  as  to  render  them  fit 
for  public  presentation.  This  secondary 
function  I  shall  endeavor,  in  the  present  in- 
stance, to  fulfil. 

I  propose  to  ta«<e  a  single  department  of 
natural  philosophy,  and  illustrate,  by  means 
of  it,  the  growth  of  scientific  knowledge  under 
the  guidance  of  experiment.  I  wish,  in  this 
fi  st  lecture,  to  make  yen  acquainted  with  cer- 
tain elementary  phenomena  ;  then  to  point 
out  to  you  how  those  theoretic  principles  by 
which  phenomena  are  explained,  take  root,  i 
and  flourish  in  the  human  mind,  and  after-  ! 
wards  to  apply  these  principles  to  the  whole 
b<-dy  of  knowledge  covered  by  the  lectures. 
The  science  of  optics  lends  itself  to  this  mode 
of  reatment,  and  on  it,  therefore,  I  propose 
to  draw  for  the  materials  of  the  present  course. 
Jt  will  be  best  to  begin  with  the  few  simple 
facts  regarding  light  which  were  known  to  the 
ancients,  and  to  pass  from  them  in  historic 
gradation  to  the  more  aLstruse  discoveries  of 
modern  times. 

All  our  notions  of  Nature,  however  exalted 
or  however  grotesque,  have  some  foundation 
in  experience.  The  notion  of  personal  voli- 
tion in  Nature  had  this  basis.  In  the  fury 
and  the  serenity  of  natural  phenomena  the 
t-  iva^e  saw  the  transcript  of  his  own  varying 
moods,  and  he  accordingly  ascribed  these 
pnenoraena  to  beings  of  like  passions  with 
hhnseit,  but  vastly  transcending  him  in  power. 


Thus  the  notion  of  causality — the  assumption 
that  natural  thi  ngs  did  not  com  2  of  themselves, 
but  had  unseen  antecedents — lay  at  the  root 
of  even  the  savage's  interpretation  of  Nature. 
Out  of  this  bias  of  the  human  mind  to  seek 
fur  the  antecedents  of  phenomena  all  science 
has  sprung. 

The  development  of  man,  indeed,  is  ulti- 
mately due  to  his  interaction  with  Nature. 
Natural  phenomena  arrest  his  attention  and 
excite  his  questionings,  the  intellectual  activity 
tims  provoked  reacting  on  the  intellect  itself, 
and  adding  to  its  strength.  The  quantity  of 
power  added  by  any  single  effort  of  the  in- 
tellect may  be  indefinitely  small  ;  but  the  in- 
tegration of  innumerable  increments  of  this 
kind  has  raised  Intellectual  power  from  its 
rudiments  to  the  magnitude  it  possesses  to- 


single  increment  is  made  good  by  the  indefi- 
nite number  of  such  increments,  summed  up 
in  what  may  be  regarded  as  practically  infinite 
time. 

We  will  not  now  go  back  to  man's  first 
intellectual  gropings  ;  much  less  shall  we 
enter  upon  the  thorny  discussion  as  to  how 
the  groping  man  arose.  We  will  take  him  at 
a  certain  stage  of  his  development,  when,  by 
evolution  or  sudden  endowment,  he  became 
possessed  of  the  apparatus  of  thought  and 
the  power  of  using  it.  For  a  time — and  that 
historically  a  long  one — he  was  limited  to 
mere  observation,  accepting  what  Nature  of- 
fered, and  confining  intellectual  action  to  it. 
The  apparent  motions  of  sun  and  stars  first 
drew  towards  them  the  questionings  of  the  in- 
tellect, and  accordingly  astronomy  was  the 
first  science  developed.  Slowly,  and  with  diffi- 
culty, the  notion  of  natural  fore-es  took  root 
in  the  mind,  the  seedling  of  this  notion  being 
the  actual  observation  of  electric  and  mag- 
netic attractions.  Slowly,  and  with  difficulty, 
the  science  of  mechanics  had  to  grow  out  of 
this  notion  ;  and  slowly  at  last  came  the  full 
application  of  mechanical  principles  to  the 
motions  of  the  heavenly  bod'es.  We  trace 
the  progress  of  astronomy  through  Hip- 
parchus  and  Ptolemy;  and,  after  a  long  halt, 
through  Copernicus,  Galileo,  Tycho  Brahe, 
and  Kepler;  w  ile,  from  the  high  table-land 
of  thought  raised  by  these  men,  Newton^ 
shoots  upward  like  a  peak,  overlooking  all 
others  from  his  dominant  elevation. 

But  other  objects  than  the  motions  of  the 
stars  attracted  the  attention  of  the  ancient 
world.  Light  was  a  familiar  phenomenon, 
and  from  the  earliest  times  we  find  men's 
rr.inds  busy  with  the  attempt  to  render  some 
account  of  it.  But,  without  experiment, 
which  belongs  to  a  later  stage  of  scientific 
development,  little  progress  could  be  made  in 
this  subject.  The  ancients,  accordingly, 
were  far  less  successful  in  dealing  with  light 
than  in  dealing  with  solar  and  stellar  mo- 
tions. Still,  they  did  make  some  progress. 
They  satisfied  themselves  that  light  moved 
in  straight  lines;  they  knew,  also,  that  these 
lines  or  rays  of  light  were  reflected  from  pol- 


ished surfaces,  and  that  the  angle  of  inci- 
dence was  equal  to  the  angle  of  reflection. 
These  two  results  of  ancient  scientific  curios- 
ity constitute  the  starting-point  of  our  pres- 
ent course  of  lectures. 

But,  in  the  first  place,  it  may  be  useful  to 
say  a  few  words  regarding  the  source  of  light 
to  be  employed  in  our  experiments.  The 
rusting  of  iron  is,  to  all  intents  and  purposes, 
the  blow  burning  of  iron.  It  develops  heat, 
and,  if  the  heat  be  preserved,  a  high  temper- 
ature may  be  thus  attained.  The  destruc- 
tion of  the  first  Atlantic  cable  was  probably 
due  to  heat  di.'V(  luped  in  this  way.  Other 
metals  are  stiil  more  combustible  than  iron. 
You  may  light  stiips  of  zinc  in  a  candle- 
flame,  and  cause  them  to  burn  almost  like 


day.     In  fact,  the  indefinite  smallness  of  the    strips  of  paper.     But,  besides  combustion  in 


SIX  LECTURES  ON  LIGHT. 


the  air,  we  may  also  have  combustion  ii?  a 
liquid.  Water,  for  example,  contains  a  store 
of  oxygen  which  may  unite  with  and  consume 
a  metal  immersed  in  it.  It  is  from  this  kind 
of  combustion  that  we  are  to  derive  the  heat 
and  light  employed  in  the  present  course. 

Their  generation  merits  a  moment's  atten- 
tion. Before  you  is  an  instrument — a  small 
voltaic  battery — in  which  zinc  is  immersed  in 
a.  suitable  liquid.  Matters  are  so  arranged 
*  hat  an  attraction  is  set  up  between  the  metal 
and  the  oxygen,  actual  union,  however,  being 
in  the  first  instance  avoided.  Uniting  the 
two  ends  of  the  battery  by  a  thick  wire,  the 
attraction  is  satisfied,  the  oxygen  unites  with 
the  metal,  the  zinc  is  consumed,  and  heat,  as 
usual,  is  the  result  of  the  combustion.  A 
power,  which,  for  want  of  a  better  name,  we 
call  an  electric  current,  passes  at  the  same 
time  through  the  wire. 

Cutting  the  thick  wire  in  two,  I  unite  the 
severed  ends  by  a  thin  one.  It  glows  with  a 
white  heat.  Whence  comes  that  heat?  The 
question  is  well  worthy  of  an  answer.  Sup- 
pose in  the  first  instance,  when  the  thick  wire 
was  employed,  that  we  had  permitted  the  ac- 
tion to  continue  until  100  grains  of  zinc  were 
consumed,  the  amount  of  heat  generated  in 
the  battery  would  be  capable  of  accurte  nu- 
merical expression.  Let  the  action  now  con- 
tinue, with  this  thin  wire  glowing,  until  100 
grains  of  zinc  are  consumed.  Will  the  amount 
(.  f  heat  generated  in  the  battery  be  the  same 
as  before?  No,  it  will  be  less  by  the  precise 
amount  generated  in  the  thin  wire  outside  the 
battery.  In  fact,  by  adding  the  internal  heat 
to  the  external,  we  obtain  for  the  combustion 
of  100  grains  of  zinc  a  total  which  never  va- 
ries. By  this  arrangement,  then,  we  are  able 
to  burn  our  zinc  at  one  place,  and  to  exhibit 
the  heat  and  light  of  its  combustion  at  a  dis- 
tant place.  In  New  York,  for  example,  we 
have  our  grate  and  fuel  ;  but  the  heat  and 
'iijht  of  our  fire  may  be  made  to  appear  at 
Sun  Francisco. 

I  now  remove  the  thin  wire  and  attach  to 
the  severed  ends  of  the  thick  one  two  thin 
rods  of  coke.  On  bringing  the  rods  together 
we  obtain  a  small  star  of  light.  Now,  the 
light  to  be  employed  in  our  lectures  is  a  sim- 
ple exaggeration  of  this  star.  Instead  of 
being  produced  by  ten  cells,  it  is  produced  by 
fifty.  Placed  in  a  suitable  camera,  provided 
with  a  suitable  lens,  this  light  will  give  us  all 
the  beams  necessary  for  our  experiments. 

And  here,  in  passing,  let  me  refer  to  the 
common  delusion  that  the  w6rks  of  Nature,  j 
the  human  eye  included,  are  theoretically  per- 
fect. The  degree  of  perfection  of  any  organ 
is  determined  by  what  it  has  to  do.  Looking 
at  the  dazzling  light  from  our  large  battery, 
you  see  a  globe  of  light,  but  entirely  fail  to 
see  the  shape  of  the  coke-points  whence  the 
light  issues.  The  cause  may  be  thus  made 
clear  :  On  the  screen  before  you  is  projected 
an  image  of  the  carbon-points,  the  whole  of 
the  lens  in  front  of  the  camera  being  employed 


to  form  the  image.  It  is  not  sharp,  but  sur. 
rounded  by  a  halo  which  nearly  obliterates  it. 
This  arises  from  an  imperfection  of  the  lens, 
called  its  spherical  aberration,  due  to  the  fact 
that  the  circumferential  and  central  rays  have 
not  the  same  focus.  The  human  eye  labors 
under  a  similar  defect,  and,  when  you  looked 
at  the  naked  light  from  fifty  cells,  the  blur  of 
light  upon  the  retina  was  sufficient  to  destrov 
the  definition  of  the  retinal  image  of  the  car- 
bons. A  long  list  of  indictments  might  in- 
deed be  brought  against  the  eye — its  opacity, 
its  want  of  symmetry,  i«~s  lack  of  achroma- 
tism, its  absolute  blindness,  in  pa:t.  All 
these  taken  together  caused  an  eminent  Ger- 
man philosopher  to  say  that,  if  any  optician 
sent  him  an  instrument  so  full  of  defects,  he 
would  send  it  back  to  him  with  the  severesf 
censure.  But  the  eye  is  not  to  be  judged 
from  the  standpoint  of  theory.  As  a  practi- 
cal instrument,  and  taking  the  adjustments  by 
which  its  defects  are  neutralized  into  account, 
it  must  ever  remain  a  marvel  to  the  reflecting 
mind 

The  ancients,  as  I  have  said,  were  aware  o/ 
the  rectilineal  propagation  of  light.  They 
knew  that  an  opaque  body,  placed  between 
the  eye  and  a  point  of  light,  intecepted  the 
light  of  the  point.  Possibly  the  terms  "  ray  " 
and  "beam"  may  have  been  suggested  by 
those  straight  spokes  of  light  which,  in  cer- 
tain states  of  the  aimosphere,  dart  from  the 
sun  at  his  rising  and  his  setting.  The  recti- 
lineal propagation  of  light  may  be  illustrated 
at  home  in  th'S  way:  Make  a  small  hole  in  a 
closed  window-shutter,  before  which  stands  a 
house  or  a  tree,  and  place  within  the  dark- 
ened room  a  white-  screen  at  some  distance 
from  the  orifice.  Every  straight  ray  proceed- 
ing from  the  house  or  tree  stamps  its  color 
upon  the  screen,  and  the  sum  of  all  the  rays 
for  us  an  image  ot  the  object.  But,  as  tne 
rays  cross  each  other  at  the  orifice,  the  image 
is  inverted.  Here  we  may  illustrate  the  sub- 
ject thus:  In  front  of  our  camera  is  a  large 
opening,  closed  at  present  by  a  sheet  of  tin- 
foil. Pricking  by  means  of  a  common  sew- 
ing-needle a  small  aperture  in  the  tin-foil,  an 
inverted  image  of  the  carbon-points  starts 
forth  upon  the  screen.  A  dozen  apertures 
will  give  a  dozen  images,  a  hundred  a  hun- 
dred, a  thousand  a  thousand.  But,  as  the 
apertures  come  closer  to  each  other,  that  is  to 
say,  as  the  tin-foil  between  the  apertures  van- 
ishes, the  images  overlap  more  and  more. 
Removing  the  tin-foil  altogether,  the  screen 
becomes  uniformly  illuminated  Hence  the 
light  upon  the  screen  may  be  regarded  as  the 
overlapping  of  innumerable  images  of  the 
carbon-points.  In  like  manner  the  light 
upon  every  white  wall  on  a  cloudless  day 
may  be  regarded  as  produced  by  the  super- 
position of  innumerable  images  of  the  sun. 

The  law  that  the  angle  of  incidence  is 
equal  to  the  angle  of  reflection  is  illustrated 
in  this  simple  way:  A  straight  lath  is  placed 
as  an  index  perpendicular  to  a  small  looking- 


SIX  LECTURES  ON  LIGHT. 


glass  capable  of  rotation.  A  beam  of  light 
is  received  upon  the  glass  and  reflected  back 
upon  the  line  of  its  incidence.  Though  the 
incident  and  the  reflected  beams  pass  in 
opposite  directions,  they  do  not  jostle  or  dis- 
place each  other.  The  index  being  turned, 
the  mirror  turns  along  with  it,  and  at  each 
side  of  the  index  the  incident  and  the 
reflected  beams  are  seen  tracking  themselves 
tnrough  the  dust  of  the  room.  The  mere 
inspection  of  the  two  angles  enclosed  be- 
twe.n  the  index  and  the  two  beams  suffices 
to  show  their  equality.  The  same  simple 
apparatus  enables  us  to  illustrate  a  law  of 
great  practical  importance,  name  y,  that, 
when  a  mirror  rotates,  the  angular  velocity 
of  a  beam  reflected  from  it  is  tvvic:  that  of 
the  reflecting  mirror.  One  experiment  will 
make  this  pla:n  to  you.  The  mirror  is 
now  vertical,  and  both  the  incident  and  the 
reflected  beams  are  horizontal.  Turning  the 
mirror  through  an  angle  of  45°  the  reflected 
beam  is  vertical ;  that  is  to  say,  it  has  moved 
f)Or ,  or  through  twice  the  angle  of  the  mirror. 

One  of  the  problems  of  science,  on  which 
scientific  progress  mainly  depends,  is  to  help 
trie  senses  of  man  by  carrying  them  into  re- 
gions which  could  never  be  attained  without 
such  help.  Thus  we  arm  the  eye  with  the 
telescope  when  we  want  to  sound  the  depths 
of  space,  and  with  the  miscroscope  when  we 
want  to  explore  motion  and  structure  in  their 
infinitesimal  dimensions.  Now,  this  law  of 
angular  reflection,  coupled  with  the  fact  that 
a  beam  of  light  possesses  no  weight,  gives  us 
the  means  of  magnifying  small  motions  to  an 
extraordinary  degree.  Thus,  by  attaching 
mirrors  to  his  suspended  magnets,  and  by 
wa  ching  the  images  of  scales  reflected  from 
the  mirrors,  the  celebrated  Gauss  was  able  to 
detect  the  slightest  thrill  or  variation  on  the 
part  of  the  earth's  magnetic  force.  The  mi- 
nute elongation  of  a  bar  of  metal  by  the  mere 
warmth  of  the  hand  may  be  so  magnified  by 
this  method  as  to  cause  the  index-beam  to 
move  from  the  ceiling  to  the  floor  of  this 
room.  The  elongation  of  a  bar  of  iron  when 
it  is  magnetized  may  be  thus  demonstrated. 
By  a  similar  arrangement  the  feeble  attrac- 
tions and  repulsions  of  the  diamagnetic  force 
have  been  made  manifest;  while  in  Sir  William 
Thompson's  reflecting  galvanometer  the  prin- 
ciple receives  one  of  its  latest  applications. 

For  more  than  1,000  years  no  step  was 
taken  in  optics  beyond  this  law  of  reflection. 
The  men  of  the  Middle  Ages,  in  fact,  endeav- 
ored on  the  o,e  hand  to  develop  the  laws  of 
the  universe  out  of  their  own  consciousness, 
while  many  of  them  were  so  occupied  with 
the  concerns  of  a  future  world  that  they 
looked  with  a  lofty  scorn  on  all  things  pertain- 
ing to  this  one.  Notwithstanding  its  demon- 
strated failure  during  1,500  years  of  trial, 
there  are  still  men  among  us  who  think  the 
riddle  of  the  universe  is  to  be  solved  by  this 
appeal  to  consciousness.  And,  like  most 
people  who  support  a  delusion,  they  maintain 


theirs  warmly,  and  show  scant  respect  for 
those  who  dissent  from  their  views.*  As  re- 
gards the  refraction  of  light,  the  course  of 
real  inquiry  was  resumed  in  1100  by  an  Ara- 
bian philosopher  named  Alhazen.  Then  it 
was  taken  up  in  succession  by  Roger  Bacon, 
Vitellio,  and  Kepler.  One  of  the  most  im- 
portant occupations  of  science  is  the  deter- 
mination, by  precise  measurements,  of  the 
quantitative  relations  of  phenomena.  The 
value  of  such  measurements  depends  upon  the 
skill  and  conscientiousness  of  the  man  who 
makes  them.  Vitellio  appears  to  have  been 
both  skilful  and  conscientious,  while  Kepler's 
habit  was  to  rummage  through  the  ob.-erv.i- 
tions  of  his  predecessors,  look  at  them  in  ail 
lights,  and  thus  distill  from  them  the  princi- 
ples whieh  united  them.  He  had  done  this 
with  the  astronomical  measurements  of 
Tycho  Brahe,  and  had  extracted  from  them 
the  celebrated  "  laws  of  Kepler."  He  did  it 
also  with  the  measurements  of  Vitellio.  But 
in  the  case  of  refraction  he  was  not  success- 
ful. The  principle,  though  a  simple  one,  es- 
caped him.  It  was  firs  discovered  by  Wille- 
brod  Snell,  about  the  year  1621. 

Less  with  the  view  of  dwelling  upon  the 
phenomenon  itself  than  of  introducing  it  to 
you  in  a  form  which  will  render  intelligible 
the  play  of  theoretic  thought  in  Newton's 
mind,  I  will  show  you  the  fact  of  refraction. 
The  dust  of  the  air  and  the  turbidity  erf  a  liquid 
may  here  be  turned  to  account.  A  shallow 
circula-  vessel  with  a  glass  face,  half  filled 
with  water,  rendered  barely  turbid  by  the 
precipitation  of  a  little  mastic,  is  placed  upon 
its  edge  with  its  glass  face  vertical.  Through 
a  slit  in  the  hoop  surrounding  the  vessel  a 
beam  of  light  is  admitted.  It  impinges  upon 
the  water,  enters  it,  and  tracks  itself  through 
the  liquid  in  a  sharp,  bright  band.  Meanwhile 
the  beam  passes  unseen  through  the  air  above 
the  water,  for  the  air  is  not  competent  to 
scatter  the  light.  A  puff  of  tobacco  smoke 
into  this  space  at  once  reveals  the  track  of  the 
incident-beam.  If  the  incidence  be  vertical, 
the  beam  is  unrefracted.  If  oblique,  its  re- 
fraction at  the  common  surface  of  air  and 
water  is  rendered  clearly  visible.  It  is  also 
seen  that  reflection  accompanies  refraction, 
the  beam  dividing  itself  at  the  point  of  inci- 
dence into  a  refracted  and  a  reflected  portion. 

The  law  by  which  Snell  connected  together 
all  the  measurements  executed  up  to  his  time, 
is  this  :  Let  A  B  C  D  represent  the  outline 
of  our  circular  vessel  (Fig.  i),  A  C  being  the 
water-line.  When  the  beam  is  incident  along 
B  E,  which  is  perpendicular  to  A  C,  there  is 
no  refraction.  When  it  is  incident  along  m 
E,  there  is  refraction  :  it  is  bent  at  E  and 
strikes  the  circle  at  n.  When  it  is  incident 


*  Schelling  thus  expresses  his  contempt  for  experi- 
mental knowledge  :  "  Newton's  Optics  is  the  greatest 
illustration  of  a  whole  structure  of  fallacies,  which  in 
all  its  parts  is  founded  on  observation  and  experi- 
nent."  There  are  some  small  imitators  of 
still  in  Germany. 


SIX  LECTURES  ON  LIGHT. 


along  m'  E,  there  is  also  refraction  at  E,  the 
beam  striking  the  point  n> '.  From  the  ends 
of  the  incident  beams,  let  the  perpendiculars 
m  o,  mf  of  be  drawn  upon  B  D,  and  from  the 
ends  of  the  refracted  beams  let  the  perpen- 
diculars p  n,pf  nf  be  also  drawn.  Measure 
the  lengths  of  o  m  and  of  p  n  and  divide  the 


one  by  the  other.  You  obtain  a  certain  quo- 
tient. In  like  manner  divide  mf  o/  by  the 
corresponding  perpendicular  pf  nf  \  vou  ob- 
tain in  each  case  the  same  qiiotient.  Snell,  in 
fact,  found  this  quotient  to  be  a  constant 
quantity  for  each  particular  substance,  though 
it  varied  in  amount  from  substance  to  sub- 
stance He  called  the  quotient  the  index  of 
refraction. 

This  law  is  oue  of  the  corner-stones  of 
optical  science,  and  its  applications  to-day 
<-<re  million-fold.  Immediately  after  its  -dis- 
covery, Descartes  Applied  it  to  the  explana- 
tion of  the  rainbow.  The  bow  :s  seen  when 
the  back  is  turned  to  the  sun.  Draw  a 
straight  line  through  the  spectator's  eye  and 
the  sun,  the  bow  is  always  seen  at  the  same 
angular  distar.ee  from  this  line.  This  was 
the  great  difficulty.  Why  should  the  bow  be 
always  and  at  all  its  parts,  forty-one  degrees 
from  this  line?  Taking  a  pen  and  calculat- 
ing the  track  of  every  ray  through  a  rain- 
drop, Descartes  found  that,  at  one  particular 
angle,  the  rays  emerged  from  the  drop  almost 
parallel  to  each  other;  being  t.;us  enabled  to 
preserve  their  intensity  through  long  atmos- 
pheric distances;  at  all  other  angles  the  rays 
quitted  the  drop  divergent,  and  through  this 
divergence  became  so  enfeebled  as  to  be 
practically  .ost  to  the  eye.  The  particular 
angle  here  referred  to  was  the  foregoing 
angle  of  forty-one  degrees,  which  observa- 
tion had  proved  to  be  invariably  that  of  the 
rainbow. 

But  in  the  rainbow  a  new  phenomenon 
was  introduced — the  phenomenon  of  color. 
And  here  we  arrive  at  one  of  those  points  in 
the  history  of  science,  when  men's  labors  so 
intermingle,  that  it  is  difficult  to  assign  to 
each  worker  his  precise  meed  of  honor.  Des- 
ca.tes  was  at  the  threshold  of  the  discovery 
of  the  composition  of  solar  light.  But  he 
failed  to  attain  perfect  clearness,  and  it  is 


certain  that  he  did  net  enunciate  the  true 
law.  This  was  reserved  for  Newton,  who 
went  to  work  in  this  way :  Through  the  closed 
window-shutter  of  a  room  he  pierced  an  ori- 
fice, and  allowed  a  thin  sunbeam  to  pass 
through  it.  The  beam  stamped  a  round 
image  of  the  sun  on  the  opposite  white  wall 
of  the  room.  In  the  path  of  this  beam  New- 
ton placed  a  prism,  expecting  to  seethe  beam 
refracted,  but  also  expecting  to  see  the  image 
of  the  sun,  after  refraction,  round;  to  his 
astonishment,  it  was  drawn  out  to  an  image 
whose  length  was  five  times  its  breadth;  and 
this  image  was  divided  into  bands  of  differ- 
ent colors.  Newton  saw  immediately  that 
solar  light  was  composite,  not  simple.  His 
image  revealed  to  him  the  fact  that  some  con- 
stituents of  the  solar  light  were  more  deflect- 
ed by  the  prism  than  others,  and  he  conclud 
ed,  therefore,  that  white  solar  light  was  a 
mixture  of  lights  of  different  colors  and  of 
different  degrees  of  refrangibility. 

Let  us  reproduce  this  celebrated  experi- 
ment. On  the  screen  is  now  stamped  a  lu- 
minous disk,  which  may  stand  for  Newton's 
image  of  the  sun.  Causing  the  beam  which 
produces  the  disk  to  pass  through  a  prism, 
we  obtain  Newton's  elongated  colored  image, 
which  he  called  a  spectrum.  Newton  divided 
the  spectrum  into  seven  parts — red,  orange, 
yellow,  green,  blue,  indigo,  violet — which 
are  commonly  called  the  seven  primary  or 
prismatic  colors.  This  drawing  out  of  the 
white  light  into  its  constituent  colors  is  called 
dispersion. 

This  was  the  first  analysis  of  solar  light  by 
Newton  ;  but  the  scientific  mind  is  fond  of 
verification,  and  never  neglects  it  where  it  is 
possible.  It  is  this  stern  conscientiousness  in 
testing  its  conclusions  that  gives  adamantine 
strength  to  science,  and  renders  all  assaults 
on  it  unavailing.  Newton  completed  his 
proof  by  synthesis  in  this  way  :  The  spec- 
trum now  before  you  is  produced  by  a  glass 
prism.  Causing  the  decomposed  beam  to 
pass  through  a  second  similar  prism,  but  so 
placed  that  the  colors  are  refracted  back  and 
•eblended,  the  perfectly  white  image  of  the 
slit  is  restored.  Here,  then,  refraction  and 
dispersion  are  simultaneously  abolished.  Are 
they  always  so  ?  Can  we  have  the  one  with- 
out the  other  ?  It  was  Newton's  conclusion 
that  we  could  not.  Here  he  erred,  and  his 
error,  which  he  maintained  to  the  end  of  his 
life,  retarded  the  progress  of  optical  discovery. 
Dolland  subsequently  proved  that,  by  com- 
bining two  different  kinds  of  glass,  the  colors 
could  be  extinguished,  still  leaving  a  residue 
of  refraction,  and  he  employed  this  residue 
in  the  construction  of  achromatic  lenses — 
lenses  which  yield  no  color — which  Newton 
thought  an  impossibility.  By  setting  a  water- 
prism — water  contained  in  a  wedge-shaped 
vessel  with  glass  sides — in  opposition  to  a 
prism  of  glass,  this  point  can  be  illustrated 
before  you.  We  have  first  the  position  of  the 
unrefracted  beam  marked  upon  the  screen ; 


SIX  LECTURES  ON  LIGHT. 


then  we  produce  the  water-spectrum  ;  finally, 
by  introducing'  a  flint  glass  prism,  we  refract 
the  beam  back,  until  the  color  cisappears. 
The  image  of  the  slit  is  now  white ;  but  you 
see  that,  though  the  dispersion  is  abolished, 
the  refraction  is  not. 

This  is  the  place  to  illustrate  another  point 
bearing  upon  the  instrumental  means  em- 
ployed in  these  lectures.  Note  the  position 
ot  the  water-spectrum  upon  the  screen.  Alter- 
ing, in  no  particular,  the  wedge-shaped  ves- 
sel, but  simply  substituting  for  the  water  the 
transparent  bisulphide  of  carbon,  you  notice 
how  much  higher  the  beam  is  thrown,  and 
how  much  richer  is  the  display  of  color. 
This  will  explain  to  you  the  use  of  this  sub- 
stance in  our  subsequent  experiments. 

The  synthesis  of  white  light  may  be 
effected  in  three  ways,  which  are  now  worthy 
of  special  attention:  Here,  in  the  first  in- 
stance, we  have  a  rich  spectrum  produced  by 
a  prism  of  bisulphide  of  carbon.  One  face 
of  the  prism  is  protected  by  a  diaphragm 
with  a  longitudinal  siit,  through  which  the 
beam  passes  into  the  prism.  It  emerges  de- 
composed at  the  other  side.  I  permit  the 
colors  to  pass  through  a  cylindrical  lens, 
which  so  squeezes  them  together  as  to  pro- 
duce upon  tiie  screen  a  sharply-defined  rect- 
angular image  of  the  longitudinal  slit.  In 
that  image  the  colors  are  re-blended,  and  you 
see  it  perfectly  white.  Between  the  prism 
and  the  cylindrical  lens  may  be  seen  the 
colors  tracking-  themselves  through  the  dust 
bf  the  room.  Cutting  off  the  more  refrangi- 
ble fringe  by  a  card,  the  rectangle  is  seen 
red  ;  cutting  off  the  less  refrangiole  fringe, 
the  rectangle  is  seen  blue.  By  means  of  a 
thin  glass  prism,  I  deflect  one  portion  of  the 
colors,  and  leave  the  residual  portion.  On 
the  screen  are  now  two  colored  rectangles 
produced  in  this  way.  These  are  comple- 
mentary colors — colors  which,  by  their  union, 
produce  white.  Note  that,  by  judicious 
management,  one  of  these  colors  is  tendered 
yellow,  and  the  other  bhie.  I  withdraw  the 
thin  prism  ;  yellow  falls  upon  blue,  and  we 
have  white  as  the  result  of  their  union.  On 
Dur  way,  we  thus  abolish  the  fallacy  first 
exposed  by  Hehnholtz,  that  the  mixture  ot 
blue  and  yellow  lights  produces  green. 

Again,  restoring  the  circular  aperture,  we 
obtain  once  more  a  spectrum  like  that  of 
Newton.  By  means  of  a  lens,  we  gather  up 
these  colors,  and  build  them  together  not  to 
an  image  of  the  aperture,  but  to  an  image  of 
the  carbon  points  themselves.  Finally,  in 
virtue  of  the  persistence  of  impressions  upon 
the  retina,  by  means  of  a.  rotating  disk,  on 
which  are  spread  in  sectors  the  colors  of  the 
spectrum,  we  blend  together  the  prismatic 
colors  in  the  eye  itself,  and  thus  produce  the 
impression  of  whiteness. 

Having  unravelled  the  interwoven  con- 
stituents of  white  light,  we  have  next  to 
inquire,  What  part  the  constitution  so 
revealed  enables  this  agent  to  play  in  Nature  ? 


To  tt  we  owe  all  the  phenomena  of  color ; 
and  yet  not  to  it  alone,  for  there  must  <be  a 
certain  relationship  between  the  ultimate  par- 
ticles of  naturaJ  bodies  and  light  to  enable 
them  to  extract  from  it  the  luxuries  of  color. 
But  the  function  of  natural  bodies  is  here 
selective,  not  creative.  There  is  no  color  gen- 
erated by  any  natural  body  whatever.  Natural 
bodies  have  showered  upon  them,  in  the 
white  light  of  the  sun,  the  sum  total  of  all 
possible  colors,  and  their  action  is  limited  to 
the  sifting  of  that  total,  the  appropriating 
from  it  of  the  colors  which  really  belong  to 
them,  and  the  rejecting  of  those  which  do 
not.  It  will  fix  this  subject  in  your  minds  if 
I  say  that  it  is  the  portion  of  light  which 
they  reject,  and  not  that  which  belongs  to 
them,  that  gives  bodies  their  colors. 

Let  us  begin  our  experimental  inquiries  here 
by  asking,  What  is  the  meaning  of  blackness  ? 
Pass  a  black  ribbon  in  succession  through  the 
colors  of  the  spectrum ;  it  quenches  all. 
This  is'  the  meaning  of  blackness — it  is  the 
result  of  the  absorption  of  all  the  constituents 
of  solar  light.  Pass  a  red  ribbon  through  the 
spectrum.  In  the  red  light  the  ribbon  is  a 
vivid  red.  Why  ?  Because  the  light  that 
enters  the  ribbon  is  not  quenched  or  absorbed, 
but  sent  back  to  the  eye.  Place  the  same  rib 
bon  in  the  green  or  blue  of  the  spectrum  ;  ,t 
is  black  as  jet.  It  absorbs  the  green  ar,d 
blue  light,  and  leaves  the  space  on  which  they 
fall  a  space  of  intense  darkness.  Place  a 
green  ribbon  in  the  green  of  the  spectrum. 
It  shines  vividly  with  its  proper  color  ;  transfer 
it  to  the  red,  it  is  black  as  jet.  Here  it  ab- 
sorbs all  the  light  that  falls  upon  it,  and  offers 
mere  darkness  to  the  eye.  When  white  light 
is  employed,  the  red  sifts  it  by  quenching  the 
green,  and  the  green  sifts  it  by  quenching  the 
red,  both  exhibiting  the  residual  color.  Thus 
the  process  through  which  natural  bodies  ac- 
quire their  colors  is  a  negative  one.  The 
colors  are  produced  by  subtraction,  not  by 
addition.  This  red  glass  is  red  because  it 
destroys  all  the  more  refrangible  rays  of  the 
spectrum.  This  blue  liquid  is  blue  because  it 
destroys  all  the  less  refrangible  rays.  Both 
together  are  opaque  because  the  light  trans- 
mitted by  the  one  is  quenched  by  the  other. 
In  this  way  by  the  union  of  two  transparent 
substances  we  obtain  a  combination  as  dark 
as  pitch  to  solar  li^ht.  This  other  liquid 
finally  is  purple  because  it  destroys  the  gree.": 
and  the  yellow,  and  allows  the  terminal  colors 
of  the  spectrum  to  pass  unimpeded.  From 
the  blending  of  the  blue  and  the  red  this  gor- 
geous color  is  produced. 

These  experiments  prepare  us  for  the  fur- 
ther consideration  of  a  point  already  adverted 
to,  and  regarding  which  error  has  found  cur- 
rency for  ages.  You  will  find  it  stated  in 
books  that  blue  and  yellow  lights  mixed  to- 
gether produce  green.  But  blue  and  yellow 
have  been  ]ust  proved  to  be  complementary 
colors,  producing  white  by  their  mixture. 
The  mixture  of  blue  and  yellow  pigments  un- 


8 


SIX  LECTURES  ON  LIGHT. 


doubtedly  produces  green,  but  the  mixture  of 
pigments  is  totally  different  from  the  mixture 
of  lights.  Helmholtz,  who  first  proved  yel- 
low and  blue  to  complementary  colors,  has 
revealed  the  cause  of  the  green  in  the  case  of 
the  pig-Rents.  No  natural  color  is  pure.  A 
blue  liquid  or  a  blue  powder  permits  not  only 
the  blue  to  pass  through  it,  but  a  portion  of 
the  adjacent  green.  A  yellow  powder  is 
transparent  not  only  to  the  yellow  light,  but 
also  in  part  transparent  to  the  adjacent  green. 
Now,  when  blue  and  yellow  are  mixed  to- 
gether, the  blue  cuts  off  the  yellow,  the  orange, 
and  the  red  ;  the  yellow,  on  the  other  hand, 
cuts  off  the  violet,  the  indigo,  and  the  blue. 
Green  is  the  only  color  to  which  both  are 
transparent,  and  the  consequence  is  that, 
when  white  light  falls  upon  a  mixture  of  yel- 
low and  blue  powders,  the  green  alone  is  sent 
back  to  the  eye.  I  have  already  shown  you 
that  the  fine  blue  ammonia-sulphate  of  copper 
transmits  a  large  portion  of  green,  while  cut- 
ting off  all  the  less  refrangible  light.  A  yel- 
low solution  of  picric  acid  also  allows  the 
green  to  pass,  but  quenches  all  the  more  re- 
frangible light.  What  must  occur  when  we 
send  a  beam  through  both  liquids?  The 
green  band  of  the  spectrum  alone  remains 
upon  the  screen. 

This  question  of  absorption  is  one  of  the 
most  subtle  and  difficult  in  molecular  physics. 
\Ve  are  not  yet  in  a  condition  to  grapple  with 
it,  but  we  shall  be  by-and-by.  Meanwhile, 
-we  may  profitably  glance  back  on  the  web  of 
relations  which  these  experiments  reveal  to 
us.  We  have,  in  the  first  place,  in  solar 
light  an  agent  of  exceeding  complexity,  com- 
posed of  innumerable  constituents,  refrangi- 
ble in  different  degrees.  We  find,  secondly, 
ihe  atoms  and  molecules  of  bodies  gifted 
with  the  power  of  sifting  solar  light  in  the 
most  various  ways,  and  producing  by  this 
rifting  the  colors  observed  in  nature  and  art. 
To  do  this  they  must  possess  a  molecular 
structure  commensurate  in  complexity  with 
that  of  light  itself.  Thirdly,  we  have  :he 
human  eye  and  brain  so  organized  as  to  be 
able  to  take  in  and  distinguish  the  multitude 
of  impressions  thus  generated.  Thus,  the 
light  at  starting  is  complex;  to  sift  and  select 
ii  as  they  do  natural  bodies  must  be  complex. 
Finally,  to  take  in  the  impressions  thus  gen- 
crated,  the  human  eye  and  brain  must  be  j 
highly  complex.  Whence  this  triple  com 
plexity  ?  If  what  are  called  material  pur- 
poses were  the  only  end  to  be  served,  a  much 
simpler  mechanism  would  be  sufficient.  But, 
instead  of  simplicity — instead  of  the  princi- 
ple of  parsimony — we  have  prodigality  of  re- 
lation and  adaptation,  and  this  apparently 
for  the  sole  purpose  of  enabling  us  to  see 
things  robed  in  the  splendor  of  color.  W'ould 
it  not  seem  that  Nature  harbored  the  inten- 
tion of  educating  us  for  other  enjoyments 
than  those  derivable  from  meat  and  drink  ? 
At  all  events,  whatever  Nature  meant — and 
it  would  be  mere  pres_mpnon  to  dogmatize 


as  to  what  she  meant — we  find  ourselves  here 
as  the  issue  and  upshot  of  her  operations,  en- 
dowed with  capacities  to  enjo\  not  only  the 
materially  useful,  but  endowed  with  others  of 
indefinite  scope  and  application,  which  deal 
alone  with  the  beautiful  and  the  true. 


LECTURE  II. 

Origin  of  Physical  Theories :  Scope  of  the  Imagina- 
tion :  Newton  and  the  Emission  Theory  :  Verifica- 
tion of  Physical  Theories :  The  Luminiferous  ; 
Ether:  Wave-Theory  of  Light:  Thomas  Young: 
Fresnel  and  Arago  :  Conceptions  of  Wave-Motion: 
Interference  of  Waves :  Constitution  of  Sound- 
Waves :  Analogies  of  Sound  and  Light:  Illustra- 
tions of  Wave-Motion :  Interference  of  Sound- 
Waves :  Optical  Illustrations:  Pitch  and  Color: 
Lengths  of  the  Waves  of  Light  and  Rates  of 
Vibration  of  the  Ether-Particles:  Interference  of 
Light  :  Phenomena  which  first  suggested  the  Un- 
dulatory  Theory:  Hooke  and  the  .Colors  of  Thin 
Plates:  The  Soap-Bubble :  Newton's  Rings: 
Theory  of  "  Fits:  "  Its  Explanation  of  the  Kings  : 
Overthrow  of  the  Theory:  Colors  of  Mother-of- 
Pearl. 

WE  might  vary  and  extend  our  experi- 
ments on  light  indefinitely,  and  they  cer- 
tainly would  prove  us  to  possess  a  wonderful 
mastery  over  the  phenomena.  But  the  ves- 
ture of  the  agent  only  would  thus  be  re- 
vealed, not  the  agent  itself.  The  human 
mind,  however,  is  so  constituted  and  so  edu- 
cated as  regards  natural  things,  that  it  can 
never  rest  satisfied  with  this  outward  view  of 
them.  Brightness  and  freshness  take  pos- 
session of  the  mind  when  it  is  crossed  by  tne 
light  of  principles,  which  show  the  facts  of 
Nature  to  be  organically  connected. 

Let  us,  then,  inquire  what  this  thing  is 
that  we  have  been  generating,  reflecting,  re- 
fracting, and  analyzing. 

In  doing  this,  we  shall  learn  that  the  life 
of  the  experimental  philosopher  is  twoiold. 
He  lives,  in  his  vocation,  a  life  of  the  senses, 
using  his  hands,  eyes,  and  ears  in  his  experi- 
ments, but  such  a  question  as  that  now  before 
us  carries  him  beyond  the  margin  of  the 
senses.  He  cannot  consider,  much  less  an- 
swer, the  question,  "What  is  light?"  with- 
out transporting  himself  to  a  world  which 
undelies  the  sensible  one,  and  out  of  which, 
in  accordance  with  rigid  law,  all  optical  phe- 
nomena spring.  To  realize  this  subsensible 
world,  if  I  may  use  the  term,  the  mh,d  must 
possess  a  certain  pictorial  power.  It  has  to 
visualize  the  invisible.  It  must  be  able  to 
form  definite  images  of  the  things  which  that 
subsensiDle  world  contains  ;  and  to  say  that, 
if  such  or  such  a  state  of  things  exist  in  that 
world,  then  the  phenomena  which  appear  in 
ours  must,  of  necessity,  grow  out  of  this 
state  of  things.  If  the  picture  be  correct, 
the  phenomena  are  accounted  for  ;  a  physical 
theory  has  been  enunciated  which  unites  anu 
explains  them  all. 

This  conception  of  physical  theory  implies, 
as  you  perceive,  the  exercise  of  the  imagina- 
tion. Do  not  be  afraid  of  this  word,  which 
seems  to  render  so  many  respectable  people, 


SIX  LECTURES  ON  LIGHT. 


both  in  the  ranks  of  science  and  out  of  them, 
uncomfortable.  That  men  in  the  ranks  of 
science  should  feel  thus  is,  I  think,  a  proof 


of  elastic  collision.  The  fact  of  optical  re- 
flection certainly  occurred  as  if  Jight  consist- 
ed of  elastic  particles,  and  this  was  Newton's 


that  they  have  suffered  themselves  to  be  mis-  I  sole  justification  for  introducing  them. 

led    by   the   popular   definition    of     a   great  i      But  this  is  not  all.     In  anotln  r  important 

faculty  instead  of  observing  its  operation  in  f  particular,    also.    Newton's    conceptions    re- 


garding the  nature  of  light  were  influenced 
by  his  previous  knowledge.  He  had  been 
working  at  the  phenomena  of  eravitation 
and  had  made  himself  at  home  amid  the  oper- 
ations of  this  universal  power.  Perhaps  his 
mind  at  this  time  was  too  freshly  and  too 
deeply  imbued  with  these  notions  to  permit 
of  his  forming  an  unfettered  judgment  re- 
garding the  nature  of  light, 
as  it  may,  Newton  saw  in 

action   of    an    attractive    force    exerted 
the     light-particles.       He     carried     his 


the 
on 


Be    that 
refraction 


their  own  minds.     Without  imagination  we 

canr.ot  take  a  step  beyond  the  bourne  of  the 

mere  animal  world,  perhaps  not  even  to  the 

edge   of    this.     But,    in    speaking    thus    of 

Imagination,  I  do  not  mean  a  riotous  power 

which   deals   capriciously   with    facts,  but  a 

ivell-ordered    and   disciplined   power,    whose 

sole   function  is  to  form  conceptions  which 

the  intellect  imperatively  demands.  Imagina- 
tion thus  exercised  never  really  severs  itself 
from  the  world  of  fact.  This  is  the  store- 
house from  which  all  its  pictures  are  drawn  ; 
and  the  magic  of  its  art  consists,  not  in 
creating  things  anew,  but  in  so  changing  the 
magnitude,  position,  and  other  relations  of 
•sensible  things,  as  to  render  them  fit  for  the 
requirements  of  the  intellect  in  the  subsen- 
uble  world.* 

1  will  take,  as  an  illustration  of  this  sub- 
ject, the  case  of  Newton.  Before  he  began 
to  deal  with  light,  he  was  intimately  ac- 
quainted with  the  laws  of  elastic  collision, 
which  all  of  you  have  seen  more  or  less  per- 
fectly illustrated  on  a  billiard-table.  As  re- 
gards the  collision  of  sens'ble  masses,  New- 
ton knew  the  angle  of  incidence  to  be  equal 

to  the  angle  of  reflection,  and  he  also  knew  j  the  refraction  seen  in  our  last  lecture.  Final- 
that  experiment,  as  shown  in  our  last  lecture,  j  ty,  it  was  supposed  that  differences  of  color 
had  established  the  same  law  with  regard  to  might  be  due  to  differences  in  the  sizes  of  the 

K—U*.  T  T  „   Ai r _i    *       i   •  •  :  »  ».:^i T*I_  •       _         _     ^.i —  i- ; i     ^.i .     .  r 


conception  out  with  the  most  severe  con- 
sistency. Dropping  vertically  downwards 
towards  the  earth's  surface,  the  motion  of  a 
body  is  accelerated  as  it  approaches  the  earth. 
Dropping  in  the  same  manner  downwards  on 
a  horizontal  surface,  say  through  air  on  glass 
or  water,  the  velocity  of  the  light-particles, 
when  they  come  close  to  the  surface,  was, 
according  to  Newton,  also  accelerated.  Ap- 
proaching such  a  surface  obliquely,  he  sup- 
posed'the  particles,  when  close  to  it,  to  be 
drawn  down  upon  it,  as  a  projectile  is 
drawn  by  gravity  to  the  surface  of  the  earth. 
This  deflection  was,  according  to  Newton, 


•light.  He  thus  found  in  his  previous  knowl- 
edge the  material  for  theoretic  images.  He 
had  only  to  change  the  magnitude  of  concep- 
tions already  in  his  mind  to  arrive  at  the 
Emission  Theory  of  Light.  He  supposed 
light  to  consist  of  elastic  particles  of  incon- 
ceivable minuteness  shot  out  with  inconceiv- 
able rapidity  by  luminous  bodies.  Such  par- 
ticles impinging  upon  smooth  surfaces  were 
reflected  in  accordance  with  the  ordinary  law 


*  The  following  charming  extract,  bearing  upon 
this  point,  was  discovered  and  written  out  lor  me  by 
my  friend,  Dr.  Bence  Jones,  Hon.  Secretary  to  the 
Royal  Institution  • 

"In  every  kind  of  magnitude  there  is  a  degree  or 
sort  to  which  our  sense  is  proportioned,  the  percep- 
tion and  knowledge  of  which  is  of  the  greatest  use  to 
mankind.  The  same  is  the  groundwork  of  philoso- 
phy :  for,  though  all  sorts  and  degrees  are  equally 
the  object  of  philosophical  speculation,  yet  it  is  irom 
those  which  are  proportioned  to  sense  that  a  philoso- 
pher must  set  out  in  his  inquiries,  ascending  or  de- 


particles.  This  was  the  physical  theory  of 
light  enunciated  and  defended  by  Newton; 
and  you  will  observe  that  it  simply  consists 
in  the  transference  of  conceptions  born  in  the 
world  of  the  senses  to  a  subsensible  world. 

But,  though  the  region  of  physical  theory 
lies  thus  behind  the  world  of  senses,  the  veri- 
fications of  theory  occur  in  that  world.  Lay- 
ing the  theoretic  conception  at  the  root  of 
matters,  we  determine  by  rigid  deduction 
what  are  the  phenomena  which  must  of  neces- 
sity grow  out  of  this  root.  If  the  phenomena 
thus  deduced  agree  with  those  of  the  actual 
world,  it  is  a  presumption  in  favor  of  the 
theory.  If  as  new  classes  of  phenomena  arise 
they  also  are  found  to  harmonize  with  theo 
retic  deduction,  the  presumption  becomes  still 
stronger.  If,  finally,  the  theory  confers  pro- 
phetic vision  upon  the  investigator,  enabling 
him  to  predict  the  existence  of  phenomena 
which  have  never  yet  been  seen,  and  if  those 


scendmg  afterwards,  as  his  pursuits  may  r<  quire.  He     V  aiV.n  VHV  en'  an.a  «   l" 

does  well  indeed  to  take  his  views  from  many  points  j  predictions  be  found  on  trial  to  be  rigidly  cor- 


of  sight,  and  supply  the  defects  of  sense  by  a  well- 
regulated  imagination  ;  nor  is  he  to  be  confined  by 
any  limit  in  space  or  time  ;  but,  as  his  knowledge  of 
Nature  is  founded  on  the  observation  of  sensible 
things,  he  must  begin  with  these,  and  must  often  re- 
turn to  them  to  examine  his  progress  by  them. 
Here  is  his  secure  hold ;  and  as  he  sets  out  from 
thenc«,  so  if  he  likewise  trace  not  often  his  steps 
backwards  with  caution,  he  will  be  in  hazard  of  losing 
his  way  in  the  labyrinths  of  Nature."— (Maclaurin  : 
An  Account  of  Sir  I.  Newton's  Philosophical  Dis- 
ctverits.  Written  1728  ;  second  editivn^  1750  ;  pp. 

A 19 


rect,  the  persuasion  of  the  truth  of  the  theory 
becomes  overpowering.  Thus  working  back- 
wards from  a  limited  number  of  phenomena, 
genius,  by  its  own  expansive  force,  reaches  a 
conception  which  covers  all  the  phenomena. 
There  is  no  more  wondertul  performance  of 
the  intellect  than  this.  And  we  can  render 
no  account  of  ii.  Like  the  scriptural  gift  of 
the  Spirit,  no  man  can  tell  whence  it  cometh. 
The  passage  from  fact  to  principle  is  some- 


10 


SIX  LECTURES  ON  LIGHT. 


times  slow,  sometimes  rapid,  and  at  all  times 
a  source  of  intellectual  joy.  When  rapid,  the 
pleasure  is  concentrated  and  becomes  a  kind 
of  ecstasy  or  intoxication.  To  any  one  who 
has  experienced  this  pleasure,  even  in  a  mod- 
erate degree,  the  action  of  Archimedes  when 
he  quitted  the  bath,  and  ran  naked,  crying 
"  Eureka!"  through  the  streets  of  Syracuse, 
becomes  intelligible. 

How,  then,  did  it  fare  with  the  theory  of 
Newton,  when  the  deductions  from  it  were 
brought  face  to  face  with  natural  phenomena  ? 
To  the  mind's  eye,  Newton's  elastic  particles 
present  themselves  like  particles  of  sensible 
magnitude.  The  same  reasoning  applies  to 
both  ;  the  same  experimental  checks  exist  for 
both.  Tested  by  experiment,  then,  Newton's 
theory  was  found  competent  to  explain  many 
facts,  and  with  transcendent  ingenuity  its 
author  sought  to  make  it  account  for  all.  He 
so  far  succeeded,  that  men  so  celebrated  as 
Laplace  and  Malus,  who  lived  till  1812,  and 
Biot  and  Brewster,  who  lived  till  our  own 
time,  were  found  among  his  disciples. 

Still,  even  at  an  early  period  of  the  existence 
of  the  Emission  Theory,  one  or  two  great 
names  were  found  recording  a  protest  against 
it  ;  and  they  furnish  another  illustration  of 
the  law  that,  in  forming  theories,  the  scientific 
imagination  must  draw  its  materials  from  the 
world  of  fact  and  experience.  It  was  known 
long  ago  that  lound  is  conveyed  in  waves  or 
pu'ses  through  the  air  ;  and  no  sooner  was  this 
truth  well  housed  in  the  mind  than  it  was  trans- 
formed into  a  theoretic  conception.  It 
supposed  that  light,  like  sound,  might  also  be 
the  product  of  wave-motion.  But  what,  in 
this  case,  could  be  the  material  forming  the 
waves  ?  For  the  waves  of  sound  we  have  the 
air  of  our  atmosphere  ;  but  the  stretch  of  im- 
agination which  filled  all  space  with  a  luminif- 
erous  ether  trembling  with  the  waves  of  light 
was  so  bold  as  to  shock  cautious  minds.  In 
one  of  my  latest  conversations  with  Sir  David 
Brewster  he  said  to  me  that  his  chief  objection 
to  the  undulatory  theory  of  light  was  that  he 
could  not  think  the  Creator  guilty  of  so  clumsy 
a  contrivance  as  the  filling  of  space  with  ether 
in  order  to  produce  light.  This,  I  may  say, 
is  very  dangerous  ground,  and  the  quarrel  of 
science  with  Sir  David,  on  this  point,  as  with 
many  other  persons  on  other  points,  is,  that 
they  profess  to  know  too  much  about  the 
mind  of  the  Creator. 

This  conception  of  an  ether  was  advocated 
and  indeed  applied  to  various  phenomena  of 
optics  by  the  celebrated  astronomer,  Huy- 
ghens.  It  was  espoused  and  defended  by  the 
celebrated  mathematician,  Euler.  They  were, 
however,  opposed  by  Newton,  whose  authority 
Or  shall  I  say 
Not  quite  so. 

Newton's  preponderance  was  in  some  degree 
due  to  the  fact  that,  though  Huyghe::s  and 
Euler  were  right  in  the  main,  tiiey  did  not 
possess  sufficient  data  to  prove  themselves 
right.  No  human  authority,  however  high, 


at  -he  time  bore  them  down. 
it   was    authority   merely  ? 


can  maintain  itself  against  the  voice  of  Natuie 
speaking  through  experiment.  But  the  voice 
of  Nature  may  be  an  uncertain  voice,  through 
the  scantiness  of  data.  This  was  the  case  at 
the  period  now  referred  to,  and  at  such  a  pe- 
riod by  the  authority  of  Newton  all  antago- 
nists were  naturally  overborne. 

Still,  this  great  Emission  Theory,  which 
held  its  ground  so  long,  resembled  one  of 
those  circles  which,  according  to  your  coun- 
tryman Emerson,  the  force  of  genius  periodi- 
cally draws  round  the  operations  of  the  in- 
tellect, but  which  are  eventually  broken 
through  by  pressure  from  behind.  In  the 
year  1773  was  born,  at  Milverton,  in  Somer- 
setshire, one  of  the  most  remarkable  men  that 
England  ever  produced.  He  was  educated 
for  th  profession  of  a  phvsician,  but  was  too 
strong  to  be  tied  down  to  professional  routine. 
He  devoted  himself  to  the  study  of  natural 
philosophy,  and  became  in  all  its  departments 
a  master.  He  was  also  a  master  of  letters. 
Languages,  ancient  and  modern,  were  housed 
within  his  brain,  and,  to  use  the  words  of  his 
epitaph,  "he  first  penetrated  the  obscurity 
which  had  veiled  for  ages  the  hieroglyphics  of 
Egypt."  It  fcil  to  the  lot  of  this  man  to  dis- 
cover facts  in  optics  which  Newton's  theory 
was  incompetent  to  explain,  and  his  mind 
roamed  in  search  of  a  sufficient  theory.  He 
had  made  himself  acquainted  with  all  the 
phenomena  of  wave-motion  ;  with  all  the 
phenomena  of  sound  ;  working  successfully 
in  this  domain  as  an  original  discoverer. 
Thus  informed  and  disciplined,  he  was  pre 
pared  to  detect  any  resemblance  which  might 
reveal  itself  between  the  phenomena  of  light 
and  those  of  wave^motion.  Such  resem- 
blances he  did  detect  ;  and,  spurred  on  by 
the  discovery,  he  pursued  his  speculations  and 
his  experiments,  until  he  finally  succeeded  in 
placing  on  an  immovable  basis  the  Undulatory 
Theory  of  Light. 

The  founder  of  this  great  theory  -was 
Thomas  Young,  a  nsme,  perhaps,  unfamiliar 
to  many  of  you.  Permit  me,  by  a  kind  of 
geometrical  construction  which  I  once  em- 
ployed in  London,  to  give  you  a  notion  of  the 
magnitude  of  this  man.  Let  Newton  stand 
erect  in  his  age,  and  Young  in  his.  Draw  a 
straight  line  from  Newton  to  Youns.,,  which 
shall  form  a  tangent  to  the  heads  of  both. 
This  line  would  slope  downwards  from  New. 
ton  to  Young,  because  Newton  was  certainly 
the  taller  man  of  the  two.  But  the  slope 
would  not  be  steep,  for  the  difference  of  stat- 
ure was  not  excessive.  The  line  would  form 
what  engineers  call  a  gentle  gradient  from 
Newton  to  Young.  Place  underneath  this 
ine  the  biggest  man  born  in  the  interval 
Between  both.  He  would  not,  in  my  opinion, 
reach  the  line ;  for  if  he  did  he  would  be 
aller  intellectually  than  Young,  and  there 
\'as,  I  believe,  none  taller.  But  I  do  not 
ant  you  to  rest  on  English  estimates  of 
young;  the  German,  Helmholtz,  a  kindred 
genius,  thus  speaks  of  him  :  "  His  was  one 


SIX  LECTURES  ON  LIGHT. 


of  the  most  profound  minds  that  the  world 
has  ever  seen  ;  but  he  had  the  misfortune  to 
be  too  much  in  advance  of  his  age.  He  ex- 
cited the  wonder  of  his  contemporaries,  who, 
however,  were  unable  to  follow  him  to  the 
heights  at  which  his  daring  intellect  was 
accustomed  to  soar.  His  most  important 
ide  s  lay,  therefore,  buried  and  forgotten  in 
the  folios  of  the  Royal  Society,  until  a  new 
'generation  gradually  and  painfully  made  the 
same  discoveries,  and  proved  the  exactness 
of  his  assertions  and  the  truth  of  his  demon- 
strations." 

It  is  quite  true,  as  Helmholtz  says,  that 
voung  was  in  advance  of  his  age  ;  but  some- 
thing is  to  be  added  which  illustrates  the 
responsibility  of  our  public  writers.  For 
twenty  years  this  man  of  genius  was  quenched 
— hidden  from  the  appreciative  intellect  of 
his  countrymen — deemed  in  fact  a  dreamer, 
through  the  vigorous  audacity  of  a  writer 
who  had  then  possession  of  the  public  ear, 
and  who  in  the  Edinburgh  Review  poured 
ridicule  upon  Young  and  his  speculations. 
To  the  celebrated  Frenchmen,  Fresnel  and 
Arago,  he  was  first  indebted  for  the  restitu- 
tion of  his  rights,  for  they,  especially  Fresnel, 
remade  independently,  as  Helmholtz  says, 
and  vastly  extended  his  discoveries.  To  the 
students  of  his  works  Young  has  long  since 
appeared  in  his  true  light,  but  these  twenty 
blank  years  pushed  him  from  the  public 
mind,  which  became  in  turn  filled  with  the 
fame  of  Young's  colleague  at  the  Royal  In 
stitution,  Davy,  and  afterwards  with  the 
fame  of  Faraday.  Carlyle  refers  to  the  re- 
mark of  Novalis,  that  a  man's  self-trust  is 
enormously  increased  the  moment  he  finds 
that  others  believe  him.  If  the  opposite 
remark  be  true — if  it  be  a  fact  that  public 
disbelief  weakens  a  man's  force — there  is  no 
calculating  the  amount  of  damage  these 
twenty  years  of  neglect  may  have  done  to 
Young's  productiveness  as  an  investigator. 
It  remains  to  be  stated  thai  his  assailant  was 
Mr.  Henry  Brougham,  afterwards  Lord 
Chancellor  of  England. 

Our  hardest  work  is  now  before  us.  And, 
as  I  have  often  had  occasion  to  notice  that 
capacity  for  hard  work  depends  in  a  great 
measure  on  the  antecedent  winding  up  of  the 
will  and  determination,  I  would  call  upon 
you  to  gird  up  your  loins  for  our  coming 
labors.  If  we  succeed  in  climbing  the  hill 
which  faces  us  to-night,  our  future  efforts 
will  be  comparatively  light. 

In  the  earliest  writings  of  the  ancients  we 
find  the  notion  that  sound  is  conveyed  by  the 
air.  Aristotle  gives  expression  to  this  no- 
tion, and  the  great  architect  Vitruvius  com- 
pares the  waves  of  sound  to  waves  of  water. 
But  the  real  mechanism  of  wave-motion  was 
hidden  from  the  ancients,  and  indeed  was 
not  made  clear  until  the  time  of  Newton. 
The  ci  ntral  difficulty  of  the  subject  was,  to 
distinguish  between  the  motion  of  the  wave 
itself  and  the  motion  of  the  particles 


which  at  any  moment  constitute  the  wave. 

Stand  upon  the  sea-shore  and  observe  the 
advancing  rollers  before  they  are  distorted  by 
the  friction  of  the  bottom.  Every  wave  has 
a  back  and  a  front,  and,  if  you  clearly  seiae 
the  image  of  the  moving  wave,  you  will  see 
that  every  particle  of  water  along  the  front 
of  the  wave  is  in  the  act  of  rising,  while 
every  particle  along  its  back  is  in  the  act  of 
sinking.  The  particles  in  front  reach  in  suc- 
cession the  crest  of  the  wave,  and  as  soon  as 
the  crest  is  passed  they  begin  to  fall.  They 
then  reach  the  furrow  or  sinus  of  the  wave, 
and  can  sink  no  farther.  Immediately  after- 
wards they  become  the  front  of  the  succeed- 
ing wave,  rise  again  until  they  reach  the 
crest,  and  then  sink  as  before.  Thus,  while 
the  waves  pass  onward  horizontally,  the 
individual  particles  are  simply  lifted  up  and 
down  vertically.  Observe  a  sea-fowl,  or,  if 
you  are  a  swimmer,  abandon  yourself  to  the 
action  of  the  waves  ;  you  are  not  carried  for- 
ward, but  simply  rocked  up  and  down.  The 
propagation  of  a  wave  is  the  propagation  of 
a  form,  and  not  the  transference  of  the  sub- 
stance which  constitutes  the  wave. 

The  length  of  the  wave  is  the  distance 
from  crest  to  crest,  while  the  distance  through 
which  the  individual  particles  oscillate  is 
called  the  amplitude  of  the  oscillation.  You 
will  notice  that  in  this  description  the  parti- 
cles of  water  are  made  to  vibrate  across  the 
line  of  propagation.* 

And  now  we  have  to  take  a  step  forward, 
and  it  is  the  most  important  step  of  all.  You 
can  picture  two  series  of  waves  proceeding 
from  different  origins  through  the  same 
water.  When,  for  example,  you  throw  two 
stones  into  still  water,  the  ring- waves  pro- 
ceeding from  the  two  centres  of  disturbance 
intersect  each  other.  Now,  no  matter  how 
numerous  these  waves  may  be.  the  law  holds 
good  that  the  motion  of  every  particle  of  the 
water  is  the  algebraic  sum  of  ail  the  motions 
imparted  to  it.  If  crest  coincide  with  crest, 
the  wave  is  lifted  to  a  double  height;  if  fur- 
row coincide  with  crest,  the  motions  are  in 
opposition,  and  thei ;  sum  is  zero.  We  have 
then  still  water,  which  we  shall  learn  pres- 
ently corresponds  to  what  we  call  darkness  in 
reference  to  our  present  subject.  This  action 
of  wave  upon  wave  is  technically  called  in- 
terference, a  term  to  be  remembered. 

Thomas  Young's  fundamental  discovery  in 
optics  was  that  the  principle  of  Interference 
applied  to  light.  Long  prior  to  his  time,  an 
Italian  philosopher,  Grimaldi,  had  stated 
that,  under  certain  circumstances,  two  thin 
beams  of  light,  each  of  which,  acting  singly, 
produced  a  luminous  spot  upon  a  white  wall, 
when  caused  to  act  together,  partially 


*  I  do  not  wish  to  encumber  the  conception  here 
with  the  details  of  the  motion,  but  I  may  draw  atten- 
tion to  the  beautiful  model  of  Prsfessor  Lyman, 


water-waves. 


SIX  LECTURES  ON  LIGHT. 


quenched  each  other  and  darkened  the  spot. 
This  was  a  statement  of  fundamental  signifi- 
cance, but  it  required  the  discoveries  and  the 
genius  of  Young  to  give  it  meaning.  How 
he  did  so,  I  will  now  try  to  make  clear  to 
you.  You  know  that  air  is  compressible  ; 
that  by  pressure  it  can  be  rendered  more 
dense,  and  that  by  dilatation  it  can  be  ren- 
dered more  rare.  Properly  agitated,  a  tun- 
ing-fork now  sounds  in  a  manner  audible  to 
you  all,  and  most  of  you  know  that  the  air 
through  which  the  sound  is  passing  is  par- 
celled out  into  spaces  in  which  the  air  is  con- 
densed, followed  by  other  spaces  in  which 
the  air  is  rarefied.  These  condensations  and 
rarefactions  constitute  what  we  call  waves  of 
sound.  You  can  imagine  the  air  of  a  room 
traversed  by  u  series  of  such  waves,  and  you 
can  imagine  a  second  series  sent  through  the 
same  air,  and  so  related  to  the  first  that  con- 
densation coincides  with  condensation  and 
rarefaction  with  rarefaction.  The  conse- 
quence of  this  coincidence  would  be  a  louder 
sound  than  that  produced  by  either  system  of 
waves  taken  singly.  But  you  can  also  ima- 
gine a  state  of  things  where  the  condensa- 
tions of  the  one  system  fall  upon  the  rarefac- 
tions of  the  other  system.  In  this  case  the 
two  systems  would  completely  neutralize  each 
other.  Each  of  them,  taken  singly,  produces 
sound;  both  of  them,  taken  together,  pro- 
duce no  sound.  Thus,  by  adding  sound  to 
sound  we  produce  silence,  as  Grimaldi  in  his 
experiment  produced  darkness  by  adding 
light  to  light. 

The  analogy  between  sound  and  light  here 
at  once  flashes  upon  the  mind.  Young  gen 
eralized  this  observation.  He  discovered  a 
multitude  of  similar  cases,  and  determined 
their  precise  conditions.  On  the  assumption 
that  light  was  wave-motion,  all  his  experi- 
ments on  interference  were  explained  ;  on  the 
assumption  that  light  was.  flying  particles, 
nothing  was  explained.  In  the  time  of  Huy- 
ghens  and  Euler  a  medium  had  been  assumed 
for  the  transmission  of  the  waves  of  light  ; 
but  Newton  raised  the  objection  that,  if  light 
consisted  of  the  waves  of  such  a  medium, 
shadows  could  not  exist.  The  waves,  he 
contended,  would  bend  round  opaque  bodies 
and  produce  the  motion  of  light  behind  them, 
as  sound  turns  a  corner,  or  as  waves  of  water 
wash  round  a  rock.  It  was  proved  that  the 
bending  round  referred  to  by  Newton  actually 
occurs,  but  that  the  inflected  waves  abolish 
each  other  by  their  mutual  interference. 
Young  also  discerned  a  fundamental  differ- 
ence between  t.e  waves  of  light  and  those  of 
sound.  Could  you  see  the  air  through  which 
sound-waves  are  passing,  you  would  observe 
every  individual  particle  of  air  oscillating  to 
and  fro  in  the  direction  of  propagation. 
Could  you  see  the  ether,  you  would  also  find 
every  individual  particle  making  a  small  ex- 
cursion to  and  fro,  but  here  the  motion,  like 
that  assigned  to  the  water-particles  above  re- 
ferred to,  would  be  across  the  line  of  propa- 


gation. The  vibrations  of  the  air  are  longi- 
tudinal, the  vibrations  of  the  ether  are  trans- 
versal, 

It  is  my  desire  that  you  should  realize  with 
clearness  the  character  of  wave-motion,  both 
in  ether  and  in  air.  And,  with  this  view.  I 
bring  before  you  an  experiment  wherein  the 
air-particles  are  represented  by  small  spots  of 
light.  They  are  parts  of  a  spiral,  drawn  upon 
a  circle  of  blackened  glass,  and,  when  the  cir- 
cle rotates,  the  spots  move  in  successive  pulses 
over  the  screen.  You  have  here  clearly  set 
before  you  how  the  pulses  travel  incessantly 
forward,  while  the  particles  that  compose 
them  perform  oscillations  to  and  fro.  This 
is  the  picture  of  a  sound-wave,  in  which  the 
vibrations  are  longitudinal.  By  another  glass 
wheel,  we  produce  an  image  of  a  trans- 
verse wave,  and  here  we  observe  the  waves 
travelling  in  succession  over  the  screen,  while 
each  individual  spot  of  light  performs  an  ex- 
cursion to  and  fro  across  the  line  of  propaga- 
tion. 

Notice  what  follows  when  the  glass  wheel 
is  turned  very  quickly.  Objectively  consid- 
ered, the  transverse  waves  propagate  them- 
selves as  before,  but  subjectively  the  effect  is 
totally  changed.  Because  of  the  retention  of 
impressions  upon  the  retina,  the  spots  of  light 
simply  describe  a  series  of  parallel  luminous 
lines  upon  the  screen,  the  length  of  these 
lines  marking  the  amplitude  of  the  vibration. 
The  impression  of  wave-motion  has  totally 
disappeared. 

The  most  familiar  illustration  of  the  inter- 
ference of  sound-waves  is  famished  by  the 
beats  produced  by  two  musical  sounds  slightly 
out  of  unison.  These  two  tuning-forks  are 
now  in  perfect  unison,  and  when  they  are 
agitated  together  the  two  sounds  flow  without 
roughness,  as  if  they  were  but  one.  But,  by 
attaching  to  one  of  the  forks  a  two-cent  piece, 
we  cause  it  to  vibrate  a  little  more  slowly 
than  its  neighbor.  Suppose  that  one  of  them 
performs  101  vibrations  in  the  time  required 
by  the  other  to  perform  100,  and  suppose 
that  at  starting  the  condensations  and  rare- 
factions of  both  forks  coincide.  At  the  loist 
vibration  of  the  quickest  fork  they  will  again 
coincide,  the  quicker  fork  at  this  point  hav- 
ing gained  one  whole  vibration,  or  one  whole 
wave  upon  the  other.  But  a  little  reflection 
will  make  it  clear  that,  at  the  $oth  vibration, 
I  the  two  forks  are  in  opposition ;  here  the  one 
tends  to  produce  a  condensation  where  the 
other  tends  to  produce  a  rarefaction;  by  the 
united  action  of  the  two  forks,  therefore,  the 
sound  is  quenched,  and  we  have  a  pause  of 
silence.  This  occurs  where  one  fork  has 
gained  half  a  wave-length  upon  the  other. 
I  At  the  loist  vibration  we  have  again  coinci 
dence,  and,  therefore,  augmented  sound;  at 
|  the  isoth  vibration  we  have  again  a  quench- 
j  ing  of  the  sound.  Here  the  one  fork  is  three 
I  half-waves  in  advance  of  the  o:  her  In  gen- 
!  eral  terms,  the  waves  conspire  when  the  one 
series  is  an  even  number  of  half- wave  lengths, 


SIX  LECTURES  ON  LIGHT. 


and  they  are  destroyed  when  the  one  series  is 
an  odd  number  of  half-wave  lengths  in  ad- 
vance of  the  other.  With  two  forks  so  cir- 
cumstanced, we  obtain  those  intermittent 
shocks  of  sound  separated  by  pauses  of  si- 
lence, to  which  we  give  the  name  of  beats. 

I  new  wish  to  show  you  what  may  be 
called  the  optical  expression  of  those  beats. 
Attached  to  a  large  tuning-fork,  F  (Fig.  2), 
is  a  small  mirror,  which  shares  the  vibrations 
of  the  fork,  and  on  to  the  mirror  is  thrown  a 
thin  beam  of  light,  which  shares  the  vibra- 
tions of  the  mirror.  The  beam  reflected 
from  the  fork  is  received  upon  a  piece  of 
looking-glass,  and  thrown  back  upon  the 
screen,  where  it  stamps  itself  as  a  small  lu- 
minous disk.  The  agitation  of  the  fork  by  a 
violin-bow  converts  that  disk  into  a  band  of 
light,  and  if  yo  i  simp.y  move  your  heads  to 
and  fro  you  cause  the  image  of  the  band  to 
sweep  over  the  retina,  drawing  it  out  to  a  sin- 
uous line,  thus  proving  the  periodic  character 
of  the  motion  which  produces  it.  By  a  sweep 
of  the  looking-glass,  we  can  also  cover  the 
screen  from  side  to  side  by  a  luminous  scroll, 
in  n,  Fig  2,  the  depth  of  the  sinuosities  indi- 
cating the  amplitude  of  the  vibration. 


band  of  light  gradually  shortening  as  the  vo« 
tiori  subsides,  until,  when  the  motion  ceases, 
:  we  hare  our  luminous  disk  restored.  Weight- 
I  ing  one  of  the  forks  as  we  did  before,  with  a 
two-cent  piece,  sometimes  the  fork']  conspire, 
and  then  you  have  the  band  of  light  drawn 
out  to  its  maximum  length  ;  sometimes  they 
oppose  each  other,  and  then  you  have  thi' 
band  of  light  diminished  to  a  circle.  Thus, 
the  beats  which  address  the  ear  express  them- 
selves optically  as  the  alternate  elongation  and 
shortening  of  the  band  of  light.  If  I  move 
the  mirror  of  this  second  fork,  you  have  a 
sinuous  line,  as  before ;  but  the  sinuosities 
are  sometimes  deep,  and  sometimes  they  al- 
most disappear,  as  in  Fig.  3,  thus  expressing 
the  alternate  increase  and  diminution  of  the 
sound,  the  intensity  of  which  is  expressed  by 
the  depth  of  the  sinuosities.  To  Lissajous  w« 
owe  this  mode  of  illustration. 


Instead  of  receiving  the  beam  reflected  from 
the  fork  on  a  piece  of  looking-glass,  we  now 
receive  it  upon  a  second  mirror  attached  to  a 
second  fork,  and  cast  by  it  upon  the  screer. 
Both  forks  now  act  in  combination  upon  the 
beam.  The  disk  is  drawn  out,  as  befoie,  the 


The/»VWfc  of  a  sound  is  w 
by  the  rapidity  of  the  vibration,  *«  tLe  in  fen- 
si  ty  is  by  the  amplitude.  The  rise  of  pitch 
with  the  rapidity  of  the  impulses  may  be  illus- 
trated by  the  syren,  which  consists  of  a  per- 
forated disk  rotating  over  a  cylinder  into 
which  air  is  forced,  and  the  end  of  which  is 
also  perforated.  When  the  perforations  of 
the  disk  coincide  with  those  of  the  cylinder,  a 
puff  escapes  ;  and,  when  the  puffs  succeed 
each  other  with  sufficient  rapidity,  the  im- 
pressions upon  the  auditory  nerve  link  them, 
selves  together  to  a  continuous  musical  note. 
The  more  rapid  the  rotation  of  the  disk  the 
quicker  is  the  succession  of  the  impulses,  and 
the  higher  the  pitch  of  the  note.  Indeed,  by 


14 


SIX  LECTURES  ON  LIGHT. 


meins  of  the  syren  the  number  of  vibrations 
due  to  any  musical  no  e,  whether  it  be  that  of 
an  instrument,  of  the  human  voice,  or  of  a 
flying  insect,  may  be  accurately  determined. 


In  front  of  our  lamp  now  stands  a  very 
homely  instrument,  S,  Fig-  4,  of  this  charac- 
ter. The  perforated  disk  is  turned  by  a 
wheel  and  band,  and,  when  the  two  sets  of 
perforations  coincide,  a  series  of  spots  of 
light,  sharply  defined  by  the  lens  L,  ranged 
on  the  circumference  of  a  circle,  is  seen  upon 
the  screen.  On  slowly  turning  the  disk,  a 
flicker  is  produced  by  the  alternate  stoppage 
and  transmission  of  the  light.  At  the  same 
time  air  is  urged  into  the  syren,  and  you  hear 
a  fluttering  sound  corresponding  to  the  flick- 
ering light.  But,  by  augmenting  the  rapid- 
ity of  rotation,  the  light,  though  intercepted 
us  before,  appears  perfectly  steady,  through 
the  persistence  of  impressions  upon  the 
retina  ;  and,  about  the  time  when  the  optical 
impression  becomes  continuous,  the  auditory 
impression  becomes  equally  so ;  the  puffs 
from  the  syren  linking  themselves  then  to- 
gether to  a  continuous  musical  note,  which 
rises  in  pitch  with  the  rapidity  of  the  rota- 
tion. A  movement  of  the  head  causes  the 
image  of  the  spots  to  sweep  over  the  retina, 
producing  beaded  lines :  the  same  effect  is 
produced  upon  our  screen  by  the  sweep  of  a 
looking  glass  which  has  received  the  thin 
beams  from  the  syren. 

In  the  undulatory  theory,  what  pitch  is  to 
the  ear,  color  is  to  the  eye.  Though  never 
seen,  the  lengths  of  the  waves  of  light  have 
been  determined.  Their  existence  is  proved 
by  their  effects,  and  from  their  effects  also 
their  lengths  may  be  accurately  deduced. 
This  may,  moreover,  be  done  in  many  ways, 
and.,  when  the  different  determinations  are 


compared,  the  strictest  harmony  is  found  t« 
exist  between  them.  The  shortest  waves  of 
the  visible  spectrum  are  those  of  the  extreme 
violet  ;  the  longest,  those  of  the  extreme 
red  ;  while  the  other  colors  are  of  intermedi- 
ate pitch  or  wave-length.  The  length  of  a 
wave  of  the  extreme  red  is  such  that  it  would 
require  36,918  of  them  placed  end  to  end  to 
cover  one  inch,  while  64,631  of  the  extreme 
violet  waves  would  be  required  to  span  the 
same  distance. 

Now,  the  velocity  of  light,  in  round  num- 
bers, is  190,000  miles  per  second.  Reducing 
this  to  inches,  and  multiplying  the  number 
.hus  found  by  36,918,  we  obtain  the  number 
of  waves  of  the  extreme  red  in  190,000  miles. 
All  these  waves  enter  the  eye,  and  hit  the 
retina  at  the  back  of  the  eye  in  one  second. 
The  number  of  shocks  per  second  necessary 
to  the  production  of  the  impression  of  red  is, 
therefore,  four  hundred  and  fifty-one  millions 
of  millions.  In  a  similar  manner,  it  may  be 
found  that  the  number  of  shocks  correspond- 
ing to  the  impression  of  violet  is  seven  hun- 
dred and  eighty-nine  millions  of  millions. 
All  space  is  rilled  with  matter  oscillating  at 
such  rates.  From  every  star  waves  of  these 
dimensions  move  with  the  velocity  of  light 
like  spherical  shells  outwards.  And  in  the 
ether,  just  as  in  the  water,  the  motion  of 
every  particle  is  the  algebraic  sum  of  all  the 
separate  motions  imparted  to  it.  Still,  one 
motion  does  not  blot  the  other  out ;  or,  if 
extinction  occur  at  one  point,  it  is  atoned  for 
at  some  other  point.  Every  star  declares  by 
its  light  its  undamaged  individuality,  as  if  it 
alone  had  sent  its  thrills  through  space. 

The  principle  of  interference  applies  to  the 
waves  of  light  as  it  does  to  the  waves  of 
water  and  the  waves  of  sound.  And  the 
condLions  of  interference  are  the  same  in  all 
three.  If  two  series  of  light- waves  of  the 
same  length  start  at  the  same  moment  from 
a  common  origin,  crest  coincides  with  crest, 
sinus  with  sinus,  and  the  two  systems  blend 
together  to  a  single  system  of  double  ampli- 
tude. If  both  series  start  at  the  same  mo- 
ment, one  of  them  being,  at  starting,  a  whole 
wave-length  in  advance  of  the  other,  they 
also  add  themselves  together,  and  we  have 
an  augmented  luminous  effect.  Just  as  in 
the  case  of  sound,  the  same  occurs  when  the 
one  system  of  waves  is  any  even  number  of 
semi-undulations  in  advance  of  the  other. 
But  if  the  one  system  be  half  a  wave-length, 
or  any  odd  number  of  half  wave-lengths  in 
advance,  then  the  crests  of  the  one  fail  upon 
the  sinuses  of  the  other  ;  the  one  system,  in 
fact,  tends  to  lift  the  particles  of  ether  at  the 
precise  places  where  the  other  tends  to  depress 
them  ;  hence,  through  their  joint  action  the 
ether  remains  perfectly  still  This  stillness 
of  the  ether  is  what  we  call  darkness,  which 
corresponds,  as  already  stated,  with  a  dead 
level  in  the  case  of  water. 

It  was  said  in  our  first  lecture,  with  refer- 
ence to  the  colors  produced  by  absorption, 


SIX  LECTURES  ON  LIGHT. 


15 


tfcat  the  function  of  natural  bodies  is  selec- 
tive,  not  creative  ;  that  they  extinguish  cer- 
tain constituents  of  the  white  solar  light,  and 
appear  in  the  colors  of  the  unextinguished 
light.  It  must  at  once  flash  upon  your  minds 
that,  inasmuch  as  we  have  in  interference  an 
agency  by  which  light  may  be  self-extin- 
quished,  we  may  have  in  it  the  conditions 
for  the  production  of  color.  But  this  would 
imply  that  certain  constituents  are  quenched 
by  interference,  while  others  are  permitted  to 
remain.  This  is  the  fact  ;  and  it  is  entirely 
due  to  the  difference  in  the  lengths  of  the 
waves  of  light. 

The  subject  is  most  easily  illustrated  by  the 
class  of  phenomena  which  first  suggested  the 
undulatory  theory  to  the  mind  of  Hooke. 
These  are  the  colors  of  thin  films  of  all  kinds, 
which  are  known  as  the  colors  of  thin  plates. 
In  this  relation  no  object  in  the  world  pos- 
sesses a  deeper  scientific  interest  than  a  com- 
mon soap-bubble.  And  here  let  me  say 
emerges  one  of  the  difficulties  which  the  stu- 
dent of  pure  science  encounters  in  the  pres- 
ence of  "practical"  communities  like  those 
of  America  and  England  ;  it  is  not  to  be  ex- 
pected that  such  communities  can  entertain 
any  profound  sympathy  with  labors  which 
seem  so  far  removed  from  the  domain  of 
practice  as  many  of  the  labors  of  the  man  of 
science  are.  Imagine  Dr.  Draper  spending 
his  days  in  blowing  soap-bubbles  and  in 
studying  their  colors  !  Would  you  show  him 
the  necessary  patience,  or  grant  him  the  nec- 
essary support  ?  And  yet,  be  it  remembered, 
it  was  thus  that  Newton  spent  a  large  portion 
of  his  time  ;  and  that  on  such  experiments 
has  been  founded  a  theory,  ths  issues  of 
which  are  incalculable.  I  see  no  other  way 
for  you  laymen  than  to  trust  the  scientific  man 
with  the  choice  of  his  inquiries  ;  he  stands 
before  the  tribunal  of  his  peers,  and  by  their 
verdict  on  his  labors  you  ought  to  abide. 

Whence,  then,  are  derived  the  colors  of  the 
soap-bubble  ?  Imagine  abeam  of  white  light 
impinging  on  the  bubble.  When  it  reaches 
the  first  surface  of  the  film,  a  known  traction 
of  the  light  is  reflected  back.  But  a  large 
portion  of  the  beam  enters  the  film,  reaches 
its  second  surface,  and  is  again  in  part  re- 
flected. The  waves  from  the  second  surface 
thus  turn  back  and  hotly  pursue  the  waves 
from  the  first  surface.  And,  if  the  thickness 
of  the  film  be  such  as  to  cause  the  necessary 
retardation,  the  two  systems  of  waves  inter- 
fere with  each  other,  producing  augmented 
or  diminished  light,  as  the  case  may  be.  But, 
inasmuch  as  the  waves  of  light  are  of  different 
lengths,  it  is  plain  that,  to  produce  self-ex- 
tinction in  the  case  of  the  longer  waves,  a 
greater  thickness  of  film  is  necessary  than  in 
the  case  of  the  shorter  ones.  Different  colors, 
therefore,  appear  at  different  thicknesses  of 
the  film. 

Take  with  you  a  little  bottle  of  spirit  of 
turpentine,  and  pour  it  into  one  of  the  ponds 
in  the  Central  Park.  You  will  then  see  the 


flashing  of  those  colors  over  the  surface  of 
the  waier.  On  a  small  scale  we  produce  them 
thus  :  A  common  tea-tray  is  filled  with  water, 
beneath  the  surface  of  which  dips  the  end  of 
a  pipette.  A  beam  of  light  falls  upon  the 
water,  and  is  reflected  by  it  to  the  screen. 
Spirit  of  turpentine  is  poured  into  the  pipette; 
it  descends,  issues  from  the  end  in  minute 
drops,  which  rise  in  i  uccession  to  the  surface. 
On  reaching  it,  each  drop  spreads  suddenly 
out  as  a  film,  and  glowing  colors  imm.ediately 
flash  forth  upon  the  screen.  The  colors 
change  as  the  thickness  of  the  film  changes 
by  evaporation.  They  are  also  arranged  in 
zones  in  consequence  of  the  gradual  diminu- 
tion of  thickness  from  the  centre  outwards. 

Any  film  whatever  will  produce  these  colors. 
The  film  of  air  between  two  plates  of  window- 
glass,  squeezed  together,  exhibits  rich  fringes 
of  color.  Nor  is  even  air  necessary  ;  the 
mere  rupture  of  optical  continuity  suffices. 
Smite  with  an  axe  the  black,  transparent  ice — 
black,  because  it  is  transparent  and  of  great 
depth — under  the  moraine  of  a  glacier  ;  you 
readily  produce  in  the  interior  flaws  which  no 
air  can  reach,  and  from  these  flaws  the  colors 
of  thin  plates  sometimes  break  like  fire.  The 
colors  are  commonly  seen  in  flawed  crystals  ; 
they  are  also  formed  by  the  film  of  oxide 
which  collects  upon  molten  lead.  It  is  the 
colors  of  thin  plates  that  guide  the  tempering 
of  steel.  But  the  origin  of  most  historic  in- 
terest is,  as  already  stated,  the  soap-bubble. 
With  one  of  those  mixtures  employed  by  the 
eminent  blind  philosopher  Plateau  in  his  re- 
searches on  the  cohesion  figures  of  thin  films, 
we  obtain  in  still  air  a  bubble  twelve  or  fifteen 
inches  in  diameter.  You  may  look  at  the 
bubble  itself,  or  you  may  look  at  its  projec- 
tion upon  the  screen,  rich  colors  arranged  in 
zones  are,  in  both  cases,  exhibited.  Render- 
ing the  beam  parallel,  and  permitting  it  to 
impinge  upon  the  sides,  bottom,  and  top  of 
the  bubble,  gorgeous  fans  of  color  overspread 
the  screen,  which  rotate  as  the  beam  is  carried 
round  the  circumference  of  the  bubble.  By 
this  experiment  the  internal  motions  of  the 
film  are  also  strikingly  displayed. 

Newton  sought  to  measure  the  thickness  of 
he  bubble  corresponding  to  each  of  these 
colors  ;  in  fact,  he  sought  to  determine  gen- 
erally the  relation  of  color  to  thickness.  His 
first  care  was  to  obtain  a  film  of  variable  and 
calculable  depth.  On  a  plano-convex  glass 
ens  of  very  feeble  curvature  he  laid  a  plate  of 
glass  with  a  plane  surface,  thus  obtaining  a 
ihn  of  air  of  gradually  increasing  depth  from 
the  point  of  contact  outwards.  On  looking  at 
he  film  in  monochromatic  light  he  saw  sui- 
-ounding  the  place  of  contact  a  series  of  bright 
ings  separated  from  each  other  by  dark  ones, 
and  becoming  more  closely  packed  together  as 
the  distance  from  the  point  of  contact  aug- 
mented. When  he  employed  red  light,  his 
ings  had  certain  diameters  ;  when  he  em- 
ployed blue  light,  the  diameters  were  less. 
Causing  his  glasses  to  pass  through  the  spev 


SIX  LECTURES  ON  LIGHT- 


trurn  from  red  to  blue,  the  rings  contracted  ; 
when  the  passage  was  from  blue  to  red,  the 
rings  expanded.  When  white  light  fell  upon 
the  glasses,  inasmuch  as  the  colors  were  not 
superposed,  a  series  of  iris-colored  circles  were 
obtained.  They  became  paler  as  the  film  be- 
came thicker,  until  finally  ihe  colors  became 
so  intimately  reblended  as  to  produce  white 
light.  A  magnified  image  of  Newton's  rings 
is  now  before  you,  and,  by  employing  in  suc- 
cession red,  blue,  and  white  lig"ht,  we  obtain 
all  the  effects  observed  by  Newton. 

He  compared  the  tints  thus  obtained  v  ith 
the  tints  of  the  soao-bubble,  and  he  calcu- 
lated the  corresponding  thickness.  How  he 
did  this  may  be  thus  made  plain  to  you  : 
Suppose  the  water  of  the  ocean  to  be  abso- 
lutely smooth  ;  it  would  then  accurately  repre- 
sent the  earth's  curved  surface.  Let  a  per- 
fectly horizontal  plane  touch  the  surface  at 
any  point.  Knowing  the  earth's  diameter, 
any  engineer  or  mathematician  in  this  room 
could  tell  you  how  far  the  sea's  surface  will 
lie  below  this  plane,  at  the  distance  of  a  yard, 
ten  yards,  a  hundred  yards,  or  a  thousand 
yards  from  the  point  of  contact  of  the  plane 
and  the  sea.  It  is  common,  indeed,  in  lev- 
elling operations,  to  allow  for  the  curvature 
of  the  earth.  Newton's  calculation  was  pre- 
cisely similar.  His  plane  glass  was  a  tan- 
gent to  his  curved  one  From  its  refractive 
index  and  focal  distance  he'  determined  the 
diameter  of  the  sphere  of  which  his  curved 
glass  formed  a  segment,  he  measured  the 
distances  of  his  rings  from  the  place  of  con- 
tact, and  he  calculated  the  depth  between  the 
tangent  plane  and  the  curved  surface,  exactly 
as  the  engineer  would  calculate  the  distance 
between  his  tangent  plane  and  the  surface  of 
the  sea.  The  wonder  is,  that,  where  such 
infinitesimal  distances  are  involved,  Newton, 
with  the  means  at  his  disposal,  couid  have 
worked  with  such  marvellous  exactitude. 

To  account  for  these  rings  was  the  great- 
est difficulty  that  Newton  ever  encoun- 
tered. He  quite  appreciated  the  difficulty. 
Over  his  eagle-eye  there  was  no  film — no 
vagueness  in  his  conceptions.  At  the  very 
outset  his  theory  was  confronted  by  the  ques- 
tion, Why,  when  a  beam  of  light  is  incident 
on  a  transparent  body,  are  some  of  the  light- 
particles  reflected  and  some  transmitted  ?  Is 
it  that  there  are  two  kinds  of  particles,  the 
one  specially  fitted  for  transmission  and  the 
other  for  reflection  ?  This  cannot  be  the 
reason  ;  for,  if  we  allow  a  beam  of  light 
which  has  been  reflected  from  one  piece  of 
glass  to  fall  upon  another,  it,  as  a  general 
rule,  is  also  divided  into  a  reflected  and  a 
transmitted  portion.  Thus  the  particles  once 
reflected  are  not  always  reflected,  nor  are  the 
particles  once  transmitted  always  transmitted. 
Newton  saw  all  this  ;  he  knew  he  had  to  ex 
plain  why  it  is  that  the  self-same  particle  is 
at  one  moment  reflected  and  at  the  next  mo- 
ment transmitted.  It  could  only  be  through 
change  in  the  condition  of  the  particle 


itself.  The  self-same  particle,  he  affirmed, 
was  affected  by  "  fits"  of  easy  transmission 
and  reflection. 

If  you  are  willing  to  follow  me  while  I  un- 
ravel this  theory  of  fits,  the  most  subtle,  per- 
haps, that  ever  entered  the  human  mind,  the 
intellectual  discipline  will  repay  you  for  the 
necessary  effort  of  attention.     Newton   was 
chary  of  stating  what  he  considered  to  be  the 
cause  of  the  fits,  but  there  cannot  be  a  doubt 
that  his  mind  rested  on  a  mechanical  cause. 
Nor  can   there  be   a  doubt  that,    as    in    all 
attempts  at  theorizing,  he  was  compelled  to 
fall  back  upon  experience  for  the  materials  of 
his  theory.     His  course  of  observation  ana 
of   thought   may   have   been   this  :    From   a 
magnet  he  might  obtain  the  notion  of   at- 
tracted    and    repelled    poles.      What    more 
natural  than  that  he  should  endow  his  light- 
particles   with    such   poles  ?      Turning  their 
attracted   poles   towards   a  transparent   sub- 
stance, the  particles  would  be  sucked  in  and 
transmitted ;    turning  their    repelled    poles, 
they   would   be   driven    away    or    reflected. 
Thus,  by  the  ascription  of  poles,  the  trans- 
mission and  reflection  of  the  self-same  parti- 
cle at  different  times  might  be  accounted  for. 
Regard  these  rings  of  Newton  as  seen  in 
pure  red   light :  they   are   alternately  bright 
and  dark.     The  film  of  air  corresponding  to 
the  outermost  of  them  is  not  thicker  than  an 
ordinary  soap-bubble,  and  it  becomes  thinner 
on  approaching  the  centre  ;  still  Newton,  a? 
I  have   said,  measured  the   thickness  corre- 
sponding to  every  ring  and  -howed  the  differ- 
ence of   thickness   between   ring   and    ring. 
Now,  mark  the  result.     For  the  sake  of  con- 
venience, let  us  call  the  thickness  of  the  film 
of  air  corresponding  to  the  first  dark  ring  d, 
then  Newton  found  the  distance  correspond- 
ing to  the   second  dark  ring  2  d;  the  thick 
ness  corresponding  to  the  third  dark  ring  3  d; 
the  thickness  corresponding  to  the  tenth  dark 
ring  10  d,  and  so  on.     Surely  there  must  be 
some  hidden  meaning  in  this  little  distanced, 
which  turns  up  so  constantly  ?     One  can  im- 
agine the  intense  interest  with  which  Newtoi* 
pondered  its  meaning.     Observe  the  probably 
outcome  of  his   thought.     He  had  endowed 
his  light-particles  with   poles,  but   now  he  is 
forced  to  introduce  the    notion  of  periodic  re- 
currence.    How  was   this    to  be  done  ?     JRy 
supposing    the  light-particles  animated,  not 
only    with     a    motion  of     translation,    but 
also  with  a   motion  of   rotation.     Newton's 
astronomical  knowledge  would  render  all  such 
conceptions  familiar  to  him.     The  earth  has 
such  a  motion.     In  the  time  occupied  in  pass- 
ing over  a  million  and  a  half  of  miles  of  its 
orbit — that    is     in    twenty  four    hours — our 
planet  performs  a  complete  rotation,  and,  in 
the  time  required  to  pass  over  the  distance  d, 
Newton's  light-particle  must  be  supposed  to 
perform    a    complete    rotation.     True,     the 
light-particle   is  smaller  than  the  planet    and 
the  distance  d.  instead  of  being  a  n?i!!icm.  snd 
half   of   miles,  is  a   little  over  the  ninety-. 


SIX  LECTURES  ON  LIGHT. 


17 


thousandth  of  an  inch.  But  the  two  con- 
ceptions are,  in  point  of  intellectual  quality, 
identical. 

Imagine,  then,  a  particle  entering  the  film 
of  air  where  it  possesses  this  precise  thick- 
ness. •.  To  enter  the  film,  its  attracted  end 
must  be  presented.  Within  the  film  it  is  able 
to  turn  once  completely  round  ;  at  the  other 
side  of  the  film  its  attracted  pole  will  be  again 
presented  ;  it  will,  therefore,  enter  the  glass 
at  the  opposite  side  of  the  film  and  be  lost  to 
the  eye.  All  round  the  place  of  contact, 
wherever  the  film  possesses  this  precise  thick- 
ness, the  light  will  equally  disappear — we 
shall  have  a  ring  of  darkness. 

And  now  observe  how  well  this  conception 
falls  in  with  the  law  of  proportionality  dis- 
covered by  Newton.  When  the  thickness  of 
the  film  is  2  d,  the  particle  has  time  to  per- 
form 

film  ;  when  the  thickness  is  3  dy  three  com 
plete  somersaults  ;  when  10  d,  ten  complete 
somersaults  are  performed.  It  is  manifest 
that  in  each  of  these  cases,  on  arriving  at  the 


sumed  that  the  action  which  produces  the  al- 
ternate bright  and  dark  rings  took  place  at  a 
single  siirface  ;  that  is,  the  second  surface  of 
the  film.  The  undulatory  theory  affirms 
that  the  rings  are  caused  by  fie  interference 
of  waves  reflected  from  both  surfaces.  This 
also  has  been  demonstrated  by  experiment. 
By  proper  devices  we  may  abolish  reflection 
from  one  of  the  surfaces  of  the  film,  and 
when  this  is  done  the  rings  vanish  altogether. 

Rings  of  feeble  intensity  are  also  formed  by 
transmitted  light.  These  are  referred  by  thft 
undulatory  theory  to  the  interference  of, 
waves  which  have  passed  directly  through  the 
film,  with  others  w;.ich  have  suffered  two  re- 
/lections  within  the  film.  They  are  thus  com- 
pletely accounted  for. 

Newton,  by  the  foregoing  exceedingly 
subtle  assumption,  vaulted  over  the  difficulty 

two   complete  somersaults   within  the  I  presented  by  the  colors  of  thin  plates.     And, 

as  further  difficulties  in  process  of  time  thick- 
ened round  the  theory,  his  disciples  tried  to 
sustain  it  with  an  ingenuity  worthy  of  their 
master.  The  new  difficulties  were  not  an- 


second  surface  of  the  film,  the  attracted  pole 
of  the  particle  will  be  presented.  It  will, 
therefore,  be  transmitted,  and,  because  no 
light  is  sent  to  the  eye,  we  shall  have  a  ring 
of  darkness  at  each  of  these  places. 

The  bright   rings  follow  immediately  from 
the   same  conception.     They  occur  between 


the  dark  rings,  the  thicknesses  to  which  they 

corresnond    h<»incr    a1«;r»    intprmprliarp  h^tw^en  I 


ticipated  by  the  theory,  but  were  met  by  new 
assumptions,  until  at  length  the  Emission 
Theory  became  what  a  distinguished  writer 
calls  a  "  mob  of  hypotheses."  In  the  pres- 
ence of  the  phenomena  of  interference,  the 
theory  finally  broke  down,  while  the  whole  of 
these  phenomena  lie,  as  it  were,  latent  in  the 
theory  of  undulation.  Newton's 


fits,"  for 

correspond  being  also  intermediate  between  j  example,   are   immediately  translatable  into 
those  of  the  dark  ones.     Take  the  case  of  the    the  lengths  of  the  ether-waves.      We  have 
first  bright   ring.     The  thickness  of  the  film 
is  yz  d;  in  this  interval  the  rotating  particle 
can   perform  only  half   a   rotation.     When, 
therefore,  it  reaches  the  second  surface  of  the 
film,    its  repelled  pole   is  presented  ;    it  is, 
therefore,  driven   back   and  reaches  the  eye. 
At  all  distances  round  the  centre  correspond- 
i-g  to  this   thickness  the  same  effect  is  pro- 


duced, and  the  consequence  is  a  ring  of 
brightness.  The  other  bright  rings  are  sim- 
ilarly accounted  for.  At  the  second  one, 
where  the  thickness  is  l  d,  a  rotation  and 


a  half  is  performed  ;  at  the  third,  two  rota- 
tions and  a  half  ;  and  at  each  of  these  places 
the  particles  present  their  repelled  poles  to 
the  lower  surface  of  the  film.  They  are  there- 
fore sent  back  to  the  eye,  producing  the  im- 
pression of  brightness.  Here,  then,  we  hayj 
unravelled  the  most  subtle  application  that 
Newton  ever  made  of  the  Emission  Theory. 

It  has  been  stated  in  the  early  part  of  this 
lecture,  that  the  Emission  Theory  assigned  a 
greater  velocity  to  light  in  glass  and  water, 
than  in  air  or  stellar  space.  Here  it  was  at 
direct  issue  with  the  theory  of  undulation, 
which  makes  the  velocity  in  air  or  stellar 
space  less  than  in  glass  or  water.  By  an  ex- 
periment proposed  by  Arago,  and  executed 
with  comsummate  skill  by  Foucault  and 
Fizeau,  this  question  was  b  -ought  to  a  crucial 
test,  and  decided  in  favor  of  the  theory  of 
undulation.  In  the  present  instance  also  the 
two  theories  are  at  variance.  Newton  as- 


the  observed  periodic  recurrence  as  the  thick- 
ness varies  so  as  to  produce  a  retardation  of 
an  odd  or  even  number  of  semi-undulations.* 
Numerous  other  colors  are  due  to  interfer- 
ence. Fine  scratches  drawn  upon  glass  or 
polished  metal  reflect  the  waves  of  light  from 
their  sides;  and  some,  being  reflected  from 
opposite  sides  of  the  same  furrow,  interfere 
with  and  quench  each  other.  But  the  ob- 
liquity of  reflection  which  extinguishes  the 
shorter  waves  does  not  extinguish  the  longer 
ones,  hence  the  phenomena  of  color.  These 
are  called  the  colors  of  striated  surfaces. 
They  are  well  illustrated  by  mother-of-pearl. 
This  shell  is  composed  of  exceedingly  thin 
layers,  which,  when  cut  across  by  the  polish- 
ing of  the  shell,  expose  their  edges  and  fur- 
nish the  necessary  small  and  regular  grooves. 
The  most  conclusive  proof  that  the  colors  are 
due  to  the  mechanical  state  of  tr.e  surface  is 
to  be  found  in  the  fact,  established  by  Brew- 
ster,  that,  by  stamping  the  shell  carefullv 


*  In  the  explanation  of  Newton's  rings,  something 
besides  thickness  is  to  be  taken  into  account.  In  the 
case  of  the  first  surface  of  the  film  of  air,  the  waves 
pass  from  a  denser  to  a  rarer  medium,  while  in  the 
case  of  the  second  surface,  the  waves  pass  from  a 
rarer  to  a  denser  medium.  This  difference  at  the 
two  reflecting  surfaces  can  be  proved  to  be  equivalent 
to  the  addition  of  half  a  -wave-length,  to  the  thick- 
ness of  the  film.  To  the  absolute  thickness,  as  de- 
termined by  Newton,  half  a  wave-length  is  in  each 
case  to  be  added.  When  this  is  done,  the  dark  and 
bright  rings  follow  each  other  in  exact  accordance 
with  the  law  of  interference  already  enunciated. 


SIX  LECTURES  ON  LIGHT. 


upon  black  sealing-wax,  we  transfer  the 
grooves,  and  produce  upon  the  wax  the  colors 
of  -nother-of-pearl. 


LECTURE  III. 

Relation  of  Theories  to  Experience :  Origin  of  the 
Notion  of  the  Attraction  of  Gravitation  :  Notion 
of  Polarity,  how  generated:  Atomic  Polarity: 
Structural  Arrangements  due  to  Polarity  :  Archi- 
tecture of  Crystals  considered  as  an  Introduction  to 
their  Action  upon  Light:  Notion  of  Atomic  Po- 
larity applied  to  Crystalline  Structure :  Experi- 
mental Illustrations:  Crystallization  of  Water: 
Expansion  by  Heat  and  by  Cold  :  Deportment  of 
VVater  considered  and  explained:  Molecular  Ac- 
tion illustrated  by  a  Model:  Force  of  Solidifica- 
tion: Bearings  of  Crystallization  on  Optical  Phe- 
nomena :  Refraction  :  Double  Refraction  :  Po- 
larization:  Action  of  Tourmaline:  Character  of 
the  Beams  emergent  from  Iceland  Spar  :  Polariza- 
tion by  ordinary  Refraction  and  Reflection :  De- 
polarization. 

IN  our  last  lecture  we  sought  to  familiarize 
our  minds  with  the  characteristics  of  wave- 
moiion.  We  drew  a  clear  distinction  between 
the  motio  >  of  the  wave  itself  and  the  motion 
of  its  constituent  particles.  Passing  through 
water-waves  and  air-waves,  we  prepared  our 
mi  ds  for  the  conception  of  light-waves  prop- 
agated through  the  luminiferous  ether.  The 
analogy  of  sound  will  fix  the  whole  mechan- 
ism in  your  minds.  Here  we  have  a  vibrat- 
vig  body  which  originates  the  wave  motion, 
•,ve  have,  in  the  air,  a  vehicle  which  conveys 
it,  and  we  have  the  auditory  nerve  which  re- 
ceives the  impressions  of  the  sonorous  waves. 
In  the  case  of  light  we  have  in  the  vibrating 
atoms  of  the  luminous  body  the  originators  of 
the  wave-motion,  we  have  in  the  ether  its 
vehicle,  while  the  optic  nerve  receives  the  im- 
pression of  the  luminiferous  waves.  We 
learned,  also,  that  color  is  the  analogue  of 
pitch,  that  the  rapidity  of  atomic  vibration 
augmented,  and  the  length  of  the  ether-waves 
decreased,  in  passing  from  the  red  to  the  blue 
end  of  the  spectrum.  The  fruitful  principle 
of  interference  we  also  found  applicable  to 
the  phenomena  of  light  ;  and  we  learned  that, 
in  consequence  of  the  different  lengths  of  the 
ether-waves,  they  were  extinguished  by  dif- 
ferent thicknesses  of  a  transparent  film,  the 
particular  thickness  which  quenched  one  color 
glowing,  therefore,  with  the  complementary 
one.  Thus  the  colors  of  thin  plates  were  ac- 
counted for. 

But  one  of  the  objects  of  our  last  lecture, 
and  that  not  the  least  important,  was  to  illus- 
trate the  manner  in  which  scientific  theories 
are  iormed.  They,  in  the  first  place,  take 
their  rise  in  the  desire  of  the  mind  to  pene- 
trate tocthe  sources  of  phenomena.  This  de- 
sire has  long  been  a  part  of  human  nature. 
It  prompted  Caesar  to  say  that  he  would  ex- 
change his  victories  for  a  Himpse  of  the 
sources  of  the  Nile  ;  it  may  be  seen  working 
in  Lucretius  ;  it  impels  Darwin  to  those  dar- 
fag  speculations  which  of  late  years  have  so 
ajrtnted  the  public  mind.  We  have  learned 
tb.*i  '  framing  theories  the  imagination  does 


not  create,  but  that  it  expands,  diminishes, 
moulds,  and  refines,  as  the  case  may  be, 
mater  als  derived  from  the  world  of  fact  and 
observation. 

This  is  more  evidently  the  case  in  a  theory 
like  that  of  light,  where  the  motions  of  a  sub- 
sensible  medium,  the  ether,  are  presented  to 
the  mind.  But  no  theory  escapes  the  condi- 
tion. Newton  took  care  not  to  encumber 
gravitation  with  unnecessary  physical  concep- 
tions ;  but  we  have  reason  to  know  that  he 
indulged  in  them,  though  he  did  not  connect 
them  with  his  theory.  But  even  the  theory 
as  it  stands  did  not  enter  the  mind  as.'  a  reve- 
lation dissevered  from  the  world  of  experi- 
ence. The  germ  of  the  conception  that  the 
sun  and  planets  are  held  together  by  a  force 
of  attraction  is  to  be  found  in  the  fact  that  a 
magnet  had  been  previously  seen  to  attract 
iron.  The  notion,  of  matter  attracting  matter 
came  thus  from  without,  not  from  within.  In 
our  present  lecture  the  magnetic  force  must 
serve  us  •  till  further  ;  but  here  we  must  master 
its  elementary  phenomena. 

The  general  facts  of  magnetism  are  most 
simply  illustrated  by  a  magnetized  bar  of 
steel,  commonly  called  a  bar  magnet.  Placing 
such  a  magnet  u~  Vrht  upon  a  table,  and 
bringing  a  magne  needle  near  its  bottom, 
one  end  of  the  n*?'-"*:  promptly  retreats  from 
the  magnet,  v  *  the  other  as  prom  pi  ly 
approaches.  leedle  is  held  quivering 

there  by  some  ..x  visible  influence  exerted 
upon  it.  Raising  the  needle  along  the  mag- 
net, but  still  avoiding  contact  the  rapidity 
of  its  oscillations  decreases,  because  the  force 
acting  upon  it  becomes  weaker.  At  the 
centre  the  oscillations  cease.  Above  the 
centre,  the  end  of  the  needle  which  had  been 
previously  drawn  towards  the  magnet  re- 
treats, and  the  opposite  end  approaches.  As 
we  ascend  higher,  the  oscillations  become 
mere  violent,  because  the  force  becomes 
stronger.  At  the  upper  end  of  the  magnet, 

It 


FIG.  5. 

as  at  the  lower,  the  force  reaches  a  maximum?. 
\  UL  all  the  lower  half  of  the  ma;.- net,  froi«.l 
K  to  S  (Fig.  5),  attracts  one  end  of  the 


SIX  LECTURES  ON  LIGHT. 


19 


needle,  while  all  the  upper  half,  from  E  to 
N,  attracts  the  opposite  end.  This  double- 
ness  of  the  magnetic  force  is  called  polaritv, 
and  the  points  near  the  ends  of  the  magnet  in 
which  the  forces  seem  concentrated  «re  called 
its  pole s. 

What,  then,  will  occur  if  we  break  this 
magnet  in  two  at  the  centre  E  !  Will  each 
of  the  separate  halves  act  as  it  did  when  it 
formed  part  of  the  whole  magnet  ?  No , 
each  half  is  in  itself  a  perfect  magnet,  pos 
sessing  two  poles.  This  may  be  proved  by 
breaking  something  of  less  value  than  the 
magnet — the  steel  of  a  lady's  stays,  for  ex- 
example,  hardened  and  magnetized.  It  acts 
like  the  magnet.  When  broken,  each  half 
acts  like  the  whole  ;  and  when  these  parts 
are  again  broken,  we  have  still  the  perfect 
magnet,  possessing,  as  in  the  first  instance, 
two  poles.  Push  your  breaking  to  its  utmost 
limit ;  you  will  be  driven  to  prolong  your 


rection  of  the  needle,  and  no  other.  A  needle 
of  iron  will  answer  as  well  as  the  magnetic 
needle  ;  for  the  needle  of  iron  is  magnetized 
by  the  magnet,  and  acts  exactly  like  a  needle 
independently  magnetized. 

If  we  place  two  or  more  needles  of  iron 
near  the  magnet,  the  action  becomes  more 
complex,  for  the  the  iron  needles  are  not  only 
acted  on  by  the  magnet,  but  they  act  upon 
each  other.  And  if  we  pass  to  smaller  masses 
of  iron — to  iron  filings,  for  example — we  find 
that  they  act  substantially  as  the  needles,  ar- 
ranging themselves  in  definite  forms,  in  obe- 
dience to  the  magnetic  action. 

Placing  a  sheet  of  paper  or  glass  over  this 
bar  magnet  and  showering  iron  filings  upon 
the  paper,  I  notice  a  tendency  of  the  filings 
io  arrange  themselves  in  determinate  lines. 
They  cannot  freely  follow  this  tendency,  for 
they  are  hampered  by  the  friction  against  the 
paper,  They  are  helped  by  tapping  th* 


FIG.  6. 

N  is  the  nozzle  of  the  lamp ;  M  a  plane  mirror,  reflecting  the  beam  upwards.  At  P,  tl»e  magnets  and 
iron  tilings  are  placed  ;  L  is  a  lens  which  forms  an  image  of  the  magnets  and  filings  ;  and  R  is  a  total- 
ly-reflecting prism  which  casts  the  image,  G,  upon  the  screen. 

paper:  each  tap  releases  them  for  a  moment! 
and  enables  them  to  follow  their  bias.  Bu 
this  is  an  experiment  which  can  only  be  seen 
by  myself.  To  enable  you  to  see  it,  I  take  a 
pair  of  small  magnets  and  by  a  simple  optical 
arrangement  throw  the  images  of  the  mag- 
nets upon  the  screen.  Scattering  iron  filings 
over  the  glass  plate  to  which  the  small  magnets 
are  attached,  and  tapping  the  i^late,  you  see 
the  arrangement  of  the  iron  filings  in  those 
magnetic  curves  which  have  been  so  long 
familiar  to  scientific  men.* 


vision  beyond  that  limit,  and  to  contemplate 
this  thing  that  we  call  magnetic  polarity  as 
resident  in  the  ultimate  particle  of  the  mag- 
net. Each  atom  is  endowed  with  this  polar 
force. 

Like  all  other  forces,  this  force  of  magnet- 
ism is  amenable  to  mechanical  laws  ;  and 
knowing  the  direction  and  magnitude  of  the 
force,  we  can  predict  its  action.  Placing  a 
small  magnetic  needle  near  a  bar  magnet,  it 
takes  up  a  determinate  position.  That  posi- 
tion might  be  deduced  theoretically  from  the 
mutual  action  of  the  poles.  Moving  the 
needle  round  the  magnet,  for  each  point  of 
the  surrounding  space  there  is  a  definite  di- 


*Very  beautiful  specimens  of  these  curves  have 
b^en  recently  obtained,  and  fixed,  by  Prof.  May^r, 
of  Hoboken 


SIX  LECTURES  ON  LIGHT. 


The  aspect  of  these  curves  so  fascinated 
Faraday  that  the  greater  portion  of  his  intel- 
lectual life  was  devoted  to  pondering  ove 
them.  He  invested  the  space  through  whic 
they  run  with  a  kind  of  -materiality  ;  and  th 
probability  is,  that  the  progress  of  science  b 
connecting  the  phenomena  of  magnetisn 
with  the  luminiferous  ether,  will  prove  thes 
"  lines  of  force,"  as  Faraday  loved  to  ca 
the  magnetic  curves,  to  represent  a  conditio 
of  this  mysterious  substratum  of  all  radian 
action. 

But  it  is  not  with  the  magnetic  curves,  a 
such,  that  I  now  wish  to  occupy  your  atten 
tion  ;  it  is  their  relationship  to  theoretic  con 
ceptions  that  we  have  now  to  consider.  B) 
the  action  of  the  bar  magnet  urJon  the  needle 
we  obtain  the  notion  of  a  polar  forc^  ;  by  th 
breaking  of  the  strip  of  magnetized  steel,  w 
attain  the  notion  that  polarity  can  attach 
itself  to  the  ultimate  particles  of  matter.  The 
experiment  with  the  iron  filings  introduces  a 
new  idea  into  the  mind  ;  the  idea,  namely,  of 
structural  arrangement.  Every  pair  of  filings 
possesses  four  poles,  two  of  which  aro  attrac- 
tive and  two  repulsive.  The  attractive  poles 
approach,  the  repulsive  poles  retreat  ;  the 
consequence  being  a  certain  definite  arrange- 
ment of  the  particles  with  reference  to  each 
other. 

Now,  this  idea  of  structure,  as  produced 
by  polar  force,  opens  a  way  for  the  intellect 
into  an  entirely  new  region,  and  the  reason  you 
are  asked  to  accompany  me  into  this  region 
is,  that  our  next  inquiry  relates  to  the  action 
of  crystals  upon  light.  Before  I  speak  of 
this  action,  I  wish  you  to  realize  the  process 
of  crystalline  architecture.  Look  then  into  a 
granite  quarry,  and  spend  a  few  minutes  in 
examining  the  rock.  It  is  not  of  perfectly 
uniform  texture.  It  is  rather  an  agglomera- 
tion of  pieces,  which,  on  examination,  pre- 
sent curiously-defined  forms.  You  have  there 
what  mineralogists  call  quartz,  you  have 
felspar,  you  have  mica.  In  a  mineralogical 
cabinet,  where  these  substances  are  preserved 
separately,  you  will  obtain  some  notion  of 
their  forms.  You  will  see  there,  also,  speci- 
mens of  beryl,  topaz,  emerald,  tourmaline, 
heavy  spar,  fluor-spar,  Iceland  spar — possibly 
a  full-formed  diamond,  as  it  quitted  the  hand 
of  Nature,  not  yet  having  got  into  the  hands 
of  the  lapidary.  These  crystals,  you  will  ob- 
serve, are  put  together  according  to  law  ; 
they  are  not  chance  productions ;  and,  if 
you  care  to  examine  them  more  minutely, 
you  will  find  their  architecture  capable  of 
being  to  some  extent  revealed.  They  split 
in  certain  directions  before  a  knife-edge,  ex- 
p.sing  smooth  and  shining  surfaces,  which 
are  called  planes  of  cleavage  ;  and  by  follow- 
ing these  planes  you  sometimes  reach  an  in 
ternal  form,  disguised  beneath  the  external 
form  of  the  crystal.  Ponder  these  beautiful 
edifices  of  a  hidden  builder.  You  cannot 
help  asking  yourself  how  they  were  built  ; 
and  familiar  as  you  now  are  with  the  notion 


of  a  polar  force,  and  the  ability  of  that  force 
to  produce  structural  arrangement,  your  in- 
evitable answer  will  be,  that  those  crystals 
are  built  by  the  play  of  polar  forces  with 
which  their  ultimate  molecules  are  endowed. 
In  virtue  of  these  forces,  atom  lays  itself  to 
atom  in  a  perfectly  definite  way,  the  final 
visible  form  of  the  crystal  depending  upon 
this  play  of  its  molecules. 

Everywhere  in  Nature  we  observe  this 
tendency  to  run  into  definite  forms,  and 
nothing  is  easier  than  to  give  scope  to  this 
tendency  by  artificial  arrangements.  Dis- 
solve nitre  in  water,  and  allow  the  water 
slowly  to  evaporate;  the  nitre  remains,  and 
the  solution  soon  becomes  so  concentrated 
that  the  liquid  form  can  no  longer  be  pre- 
served. The  nitre-molecules  approach  each 
other,  and  come  at  length  within  the  range 
of  their  polar  forces.  They  arrange  them- 
selves in  obedience  to  these  forces,  a  minute 
crystal  of  nitre  being  at  first  produced.  On 
this  crystal  the  molecules  continue  to  deposit 
themselves  from  the  surrounding  liquid.  The 
crystal  grows,  and  finally  we  have  large 
prisms  of  nitre,  each  of  a  perfectly  definite 
hape.  Alum  crystallizes  with  the  utmost 
ease  in  this  fashion.  The  resultant  crystal 
s,  however,  different  in  shape  from  that  01 
nitre,  because  the  poles  of  the  molecules  are 
differently  disposed;  and,  if  they  be  only 
nursed  with  proper  care,  crystals  of  these 
substances  may  be  caused  to  grow  to  a  great 
size. 

The  condition  of  perfect  crystallization  is, 
:hat  thi  crystallizing  force  shall  act  with  de- 
iberafion.  There  should  be  no  hurry  in  its 
Deration;  but  every  molecule  ought  to  be 
)ermitted,  without  disturbance  from  its  neigh- 
>ors,  to  exercise  its  own  molecular  rights.  If 
he  crystallization  be  too  sudden,  the  regu 
arity  disappears.  Water  may  be  saturated 
with  sulphate  of  soda,  dissolved  when  the 
water  is  hot,  and  afterward  permitted  to  cool. 
When  cold,  the  solution  is  supersaturated  ; 
hat  is  to  say,  more  solid  matter  is  contained 
n  it  than  corresponds  to  its  temperature, 
till  the  molecules  show  no  signs  of  building 
hemselves  together.  This  is  a  very  remaik- 
.ble,  though  a  very  common  fact.  The 
tiolecules  in  the  centre  of  the  liquid  are  so 
lampered  by  the  action  of  their  neighbors 
hat  freedom  to  follow  their  own  tendencies 
s  denied  to  them.  Fix  your  mind's  eye  upon 

molecule  within  the  mass.  It  wishes  to 
nite  with  its  neighbor  to  the  right  but  it 
rishes  equally  to  unite  with  its  neighbor  to 
he  left ;  the  one  tendency  neutralizes  the 
ther,  and  it  unites  with  neither.  We  have 
ere,  in  fact,  translated  into  molecular  action 
ic  well-known  suspension  of  animal  volition 
reduced  by  two  equally  inviting  bundles  of 
ay.  But,  if  a  crystal  of  sulphate  of  soda  be 
ropped  into  the  solution,  the  molecular  in- 
ecision  ceases  On  the  crystal  the  adjacent 
lolecules  will  immediately  precipitate  them- 
elves;  on  these  again  others  will  be  precipi- 


SIX   LF.CTURrs  OX   LIGHT. 


tatod,  and  this  not  of  precipitation  will  con- 
tinue from  the  top  of  the  flask  to  the  bottom, 
until  the  solution  has,  as  far  as  possible,  as- 
sumed the  solid  form.  The  crystals  here 
formed  are  small,  and  confusedly  arranged. 
The  process  has  been  too  hasty  to  a  mit  of 
the  pure  and  orderly  action  of  the  crystalliz- 
ing- force.  It  typifies  the  state  of  a  nation  in 
which  natural  and  healthy  change  is  resisted, 
until  society  becomes,  as  it  were,  supersatu- 
rated with  the  desire  for  change,  the  change 
being  effected  through  confusion  and  revolu- 
tion, which  a  wise  foresight  might  have 
avoided. 

Let  me  illustrate  the  action  of  crystallizing 
force  by  two  examples  of  it  :  Nitre  might  be 
employed,  but  another  well-known  substance 
enables  me  to  make  the  experiment  in  a  bet- 
ter form.  The  substance  is  common  sal- 
ammoniac,  or  chloride  of  ammonium,  dis- 
solved in  water.  Cleansing  perfectly  a  glass 
plate,  the  solution  of  the  chloride  is  poured 
over  the  glass,  to  which,  when  the  plate  is  set 
on  edge,  a  thin  film  of  the  liquid  adheres. 
Warming  the  glass  slightly,  evaporation  is 
promoted  ;  the  plate  is  then  placed  in  a  solar 
microscope,  and  an  image  of  the  film  is  thrown 
upon  a  white  screen.  The  warmth  of  the  il- 
luminating beam  adds  itself  to  that  already 
impa-  ted  to  the  glass  plate,  so  that  after  a 
moment  or  two  the  film  can  no  longer  exist  in 
the  liquid  condition.  Molecule  then  closes 
with  molecule,  and  you  have  a  most  impres- 
sive display  of  crystallizing  energy  overspread- 
ing the  whole  screen.  You  may  produce 
something  similar  if  you  breathe  upon  the 
frost  ferns  which  overspread  your  window- 
panes  in  winter,  and  then  observe  througrh  a 
lens  the  subsequent  recongelation  of  the  film. 

Here  the  crystallizing  force  is  hampered  by 
the  adhesion  of  the  film  to  the  glass  ;  never- 
theless, the  play  of  power  is  strikingly  beau- 
tiful. Sometimes  the  crystals  start  from  the 
edge  of  the  film  and  run  through  it  from  that 
edge,  for,  the  crystallization  being  once 
started,  the  molecules  throw  themselves  by 
preference  on  the  crystals  already  formed. 
Sometimes  the  crystals  start  from  definite 
nuclei  in  the  centre  of  the  film  ;  every  small 
crystalline  particle  which  rests  in  the  film  fur- 
nishes a  starting-point.  Throughout  the  pro- 
cess you  notice  one  feature  which  is  perfectly 
unalterable,  and  that  is,  angular  magnitude. 
The  spiculse  branch  from  the  trunk,  and  from 
these  branches  others  shoot ;  but  the  angles 
enclosed  by  the  spiculae  are  unalterable.  In 
like  manner  you  may  find  alum-crystals, 
quartz-crystals,  and  all  other  crystals,  dis- 
torted in  shape.  They  are  thus  far  at  the 
mercy  of  the  accidents  of  crystallization  ;  but 
in  one  particular  they  assert  their  superiority 

over  all  such  accidents — angular  magnitude  \  parallel  to  the  planes  of  freezing,  and  send- 
is  always  rigidly  preserved.  j  ing  a  sunbeam  through  such  a  slab,  it  lique- 

My  second  example  of  the  action  of  crys-  j  fies  internally  at  special  points,  round  each 
tallizing  force  is  this:  <  y  sending  a  voltaic  !  point  a  six-petalled  liquid  flower  of  exquisite 
current  through  a  liquid,  you  know  that  we  (  beauty  being  formed.  Crowds  of  such  flow- 
decompose  the  liquid,  and  if  it  contains  a  ers  are  thus  produced. 


metal,  we  liberate  this  metal  by  the  electro- 
lysis. This  small  cell  contains  a  solution  of 
acetate  of  lead,  and  this  substance  is  chosen 
because  lead  lends  itself  freely  to  this  crys- 
tallizing power.  Into  the  cell  dip  two  very 
thin  plati.  um  wires,  and  these  are  connected 
by  other  wires  with  a  small  voltaic  battery. 
On  sending  the  voltaic  current  through  the 
solution,  the  1  ad  will  be  si  wly  severed  from 
the  atoms  with  which  it  is  now  combined;  it 
will  be  liberated  upon  one  of  the  wires,  and 
at  the  moment  of  its  liberation  it  will  obey 
the  polar  forces  of  its  atoms,  and  produce 
crystalline  forms  of  exquisite  beauty.  They 
are  now  before  you,  sprouting  like  ferns 
from  the  wire,  appearing  indeed  like  vegeta- 
ble growths  rendered  so  rapid  as  to  be  plain- 
ly visible  to  the  naked  eye.  On  reversing  the 
current,  these  wonderful  lead-fronds  will  dis- 
solve, while  from  the  other  wire  filaments  of 
lead  dart  through  the  liquid.  In  a  moment 
or  two  the  growth  of  the  lead-trees  recom- 
mences, but  they  now  cover  the  other  wire. 
In  the  process  of  crystallization,  Nature  first 
reveals  herself  as  a  builder.  Where  do  her 
operations  stop  ?  Does  she  continue,  by  the 
play  of  the  same  forces,  to  form  the  vegeta- 
ble, and  afterwards  the  animal  ?  Whatever 
the  answer  to  these  questions  may  be,  trust 
me  that  the  notions  of  the  coming  genera- 
tions regarding  this  mysterious  thing,  which 
some  have  called  "brute  matter,"  will  be' 
very  different  from  those  of  the  generations 
past. 

There  is  hardly  a  more  beautiful  and  in- 
structive example  of  this  play  of  molecular 
force  tnan  that  furnished  by  the  case  of  water. 
You  have  seen  the  exquisite  fern-like  forms 
produced  by  the  crystallization  of  a  film  of 
water  on  a  cold  window  pane.  You  have 
also  probably  noticed  the  beautiful  rosettes 
tied  together  by  the  crystallizing  force  during 
the  descent  of  a  snow-shower  on  a  very  calm 
day.  The  slopes  and  summits  of  the  Alps 
are  loaded  in  winter  with  these  blossoms  of 
the  frost.  They  vary  infinitely  in  detail  of 
beauty,  but  the  same  angular  magnitude  is 
preserved  throughout.  An  inflexible  power 
binds  spears  and  spiculse  to  the  angle  of  60 
degrees.  The  common  ice  of  our  lakes  is 
also  ruled  in  its  deposition  by  the  same  angle. 
You  may  sometimes  see  in  freezing  water 
small  crystals  of  stellar  shapes,  each  star  con- 
sisting of  six  rays,  with  this  angle  of  60°  be- 
tween every  two  of  them.  This  structure 
may  be  revealed  in  ordinary  ice.  In  a  sun- 
beam, or,  failing  that,  in  our  electric  beam, 
we  have  an  instrument  delicate  enough  to 
unlock  the  frozen  molecules  without  disturb- 
ing the  order  of  their  architecture.  Cutting 
from  clear,  sound,  regularly-frozen  ice  a  slab 


SIX  LECTURES  ON  LIGHT. 


A  moment's  further  devotion  to  the  crys- 
tallization of  water  will  be  well  repaid  ;  for 
the  sum  of  qualities  which  renders  ihis  sub- 
stance fitted  to  play  its  part  in  Nature  may 
well  excite  wonder  and  stimulate  thought. 
Like  almost  all  other  substances,  water  is  ex- 
panded by  heat  and  contracted  by  cold.  Let 
this  expansion  and  contraction  be  first  illus- 
trated : 

A  small  fla'-k  is  filled  with  colored  water, 
and  stopped  with  a  cork.  Through  the  cork 
passes  a  glass  tube  water-tight,  the  liquid 
standing  at  a  certain  height  (/',  Fig.  7)  in  the 
tube.  The  flask  and  its  tube  resemble  tho 
bulb  and  stem  of  a  thermometer.  Applying- 
the  heat  of  a  spirit-lamp,  the  water  rises  in 
the  tube,  and  finally  trickles  over  the  top  (/). 
Expansion  by  heat  is  thus  illustrated. 


the  definite  temperature  zi  39*  Fahr,     Crys, 

tallization  has  virtually  here  commenced,  the 

molecules  preparing  themselves  for  the  subse. 

quent  act  of   solidification  which  occurs  at 

32°,   and  in  which   the  expansion   suddenly 

culminates.       In  virtue   of    this   expansion, 

j  ice,  as  you  know,  is  lighter  than  water  in  the 

|  proportion  of  8  to  9.* 

It  is  my  desire,  in  these  lectures,  to  leaa 
j  you   as   closely     as   possible    to    tr.e    limits 
I  hitherto  attained  by  scientific  thought,  and, 
|  in  pursuance  rf  this  desire,  I  have  now  to 
i  invite  your  attention  to  a  molecular  problem 
I  of  great  interest,  but  of  great  complexity.     I 
j  wish  you  to  obtain  sue  i  an  insight   of  ^he 
molecular   world   as  shall   give  the  intellect 
satisfaction  when    reflecting  on    the    deport- 
ment of  water  before  and  during  the  act  of 


Projection  of  experiment :    E  is  the  nozzle  of  the  lamp, 
column. 

Removing  the  lamp  and  piling  a  freezing 
mixture  in  the  vessel  (B)  round  the  flask,  the 
liquid  column  falls,  thus  showing  the  con- 
traction of  the  water  by  the  cold.  But  let 
the  freezing  mixture  continue  to  act  :  the 
falling  of  the  column  continues  to  a  certain 
point;  it  then  ceases.  The  top  of  tie  col- 
umn remains  stationary  for  son  e  seconds, 
and  afterwards  begins  to  rise.  The  contrac- 
tion has  ceased,  and  expansion  by  cold  sets  in. 
Let  the  expansion  continue  till  the  liquid 
trickles  a  second  time  over  the  top  of  the 
tube.  The  freezing  mixture  has  here  pro- 
duced to  all  appearance  the  same  effect  as  the 
flame.  In  the  case  of  water,  contraction!  by 
cold  ceases  and  expansion  by  cold  sets  in  at 


L  a  converging  lens,  and  z"  z'  the  image  of  the  liquid 

crystallization.       Consider,    then,    the     ideal 
case  of  a  number  of   magnets    deprived    of 

*  In  a  little  volume  entitled  'l  Forms  of  Water," 
I  have  mentioned  that  cold  iron  floats  upon  molten 
iron.  lu  company  with  my  friend  Sir  William  Arm- 
strong, I  had  repeated  opportunities  of  witnessing 
this  fact  in  his  works  at  Elswick.  in  1863.  Faraday, 
I  remember,  spoke  to  me  subsequently  of  the  com- 
pleteness of  iron  castings  as  probably  due  to  trie 
swelling  of  the  metal  on  solidification.  Beyond  'his, 
Ihavegiv.-n  the  subject  no  special  attention,  and  I 
know  that  many  intelligent  iron-foundt  rs  doubt  the 
fact  of  expansion.  It  is  quite  possible  that  the  solid 
floats  because  it  is  not  -wetted  by  the  molten  iron,  its 
volume  being  virtually  augmented  by  capillary  re- 
pulsion. Certain  flies  walk  freely  upon  water  in  vir- 
tue of  an  action  of  this  kind.  With  bismuth,  how- 
ever, it  is  easy  to  burst  iron  bottles  by  the  force  of 
solidification. 


SIX  LECTURES  ON  LIGHT. 


weight,  but  retaining  their  polar  forces.  If 
we  had  a  liquid  of  the  specific  gravity  of 
steel,  we  might,  by  making  the  magnets 
float  in  it,  realize  this  state  of  things,  for  in 
s>uch  a  liquid  the  magnets  would  neither  sink 
nor  swim.  Now,  the  principle  of  gravitation 
is  that  every  particle  of  matter  attracts  every 
other  particle  with  a  force  varying  as  the  in- 
inverse  square  of  the  distance.  In  virtue  of 
the  attraction  of  gravity,  then,  the  magnets, 
if  perfectly  free  to  move,  would  slowly  ap- 
proach each  other. 

But  besides  the  unpolar  force  of  gravity, 
which  belongs  to  matter  in  general,  the  mag- 
nets are  endowed  with  the  polar  force  of 
magnetism.  For  a  time,  however,  the  polar  j 
forces  do  not  sensibly  come  into  play.  In 
thi^  condition  the  magnets  resemble  our  water 
molecules  at  the  temperature  say  of  50*. 


with  the  force  of  contraction  until  the  freezing 
temperature  is  attained.  Here  the  polar 
forces  suddenly  and  finally  gain  the  victory. 
The  molecules  close  up  and  form  solid  crys- 
tals, a  considerable  augmentation  of  volume 
being  the  immediate  consequence. 

We  can  still  further  satisfy  the  intellect  by 
showing  that  these  conceptions  can  be  real- 
ized by  a  model.  The  molecule  of  water  is 
composed  of  two  atoms  of  hydrogen,  united 
to  one  of  oxygen.  We  may  assume  the  mole- 
cide  built  up  of  these  atoms  to  be  pyramidal. 
Suppose  the  triangles  in  Fig.  8  to  be  drawn 
touching  the  sides  of  the  molecule,  and  the 
disposition  of  the  polar  forces  to  be  that  i-.di- 
cated  by  the  letters  ;  the  points  marked  A 
being  attractive,  and  those  marked  R  repel- 
lent. In  virtue  of  the  general  attraction  of 
the  molecules,  let  them  be  d»-rwn  towards  the 


,' 


But  the  magnets  come  at  length  suffici"-,ily 
near  each  other  to  enable  their  poles  to  nter- 
act.  From  this  point  the  action  ceases*  to  be 
a  general  attraction  of  the  masses,  .^n  at- 
traction of  special  points  of  the  masses  and  a 
repulsion  of  other  points  now  come  into  play; 
and  it  is  easy  to  see  that  the  rearrangement 
of  the  magnets  consequent  upon  the  intro- 
duction of  these  new  forces  may  be  such  as 
to  require  a  greater  amount  of  room.  This, 
I  take  it,  is  the  case  with  our  water-mole- 
cules. Like  the  magnets,  they  approach  each 
other  as  wholes,  until  the  temperature  ^9°  is 
reached.  Previous  to  this  temperature, 
doubtless,  the  polar  forces  had  begun  to  act, 
and  at  this  temperature  their  action  exactly 
balances  the  contraction  due  to  cold.  At 
lower  temperatures  the  polar  forces  predomi- 
nate. But  they  carry  on  a  gradual  struggle 


positions  marked  by  the  frill  lines,  and  then 
suppose  the  polar  attractions  and  repulsions 
to  act.  A  will  turn  towards  A',  and  R  will 
retreat  from  Rx.  The  molecules  will  be  caused 
to  rotate,  their  final  positions  being  that  shown 
by  the  dotted  lines.  But  the  circle  surround 
ing  the  latter  is  larger  than  that  surrounding 
the  full  lines,  which  shows  that  the  molecules 
in  their  new  positions  n  quire  more  room.  In 
this  v.  ay  we  obtain  an  image  of  the  molecular 
mechai  ism  active  in  the  case  of  water.  The 
demand  for  more  room  is  made  with  an  energy 
sufficient  to  overcome  all  ordinary  resistances. 
Your  lead  pipes  yield  readily  to  this  power;  but 
iron  does  the  same,  and  bomb-shells,  as  you 
know,  can  be  burst  by  the  freezing  of  water. 
Thick  iron  bottles  filled  with  water  and  placed 
in  a  freezing  mixture  are  shivered  into  frag- 
ments by  the  resistless  vigor  of  molecular  force. 


SIX  LECTURES  ON  LIGHT. 


We  have  now  to  exhibit  the  bearings  of 
crystallization  upon  optical  phenomena.  Ac- 
cording to  the  undulatory  theory,  the  velocity 
of  light  in  water  and  glass  is  less  than  in 
air.  Consider,  then,  a  small  portion  of  a 
wave  issuing  from  a  point  of  light  so  distant 
that  the  portion  may  be  regarded  as  practi- 
cally straight.  Moving  vertically  downwards, 
and  impinging  on  an  horizontal  surface  of 
glass,  the  wave  would  go  through  the  glass 
without  change  of  direction.  But,  as  the 
velocity  in  glass  is  less  than  the  velocity  in 
air,  the  wave  would  be  retarded  on  passing 
into  the  denser  medium. 

But  suppose  the  wave,  before  reaching  the 
glass,  to  be  oblique  to  the  surface  ;  that  end 
of  the  wave  which  first  reaches  the  glass  will 
be  the  first  retarded,  the  other  portions  as 
they  enter  the  glass  being  retarded  in  succes- 
sion. This  retardation  of  the  one  end  of  the 
wave  causes  it  to  swing  round  and  change  its 
front,  so  that  when  the  wave  has  fully  entered 
the  glass  its  course  is  oblique  to  its  original  di- 
rection. According  to  the  undulatory  theory, 
light  is  thus  refracted. 


In  water,  for  example,  there  is  nothing  in 
the  grouping  of  the  molecules  to  interfere 
with  the  perfect  homogeneity  of  the  ether ; 
but,  when  water  crystallizes  to  ice,  the  case 
is  different.  In  a  plate  of  ice  the  elasticity 
of  the  ether  in  a  direction  perpendicular  to 
the  surface  of  freezing  is  different  from  what 
it  is  parallel  to  the  surface  of  freezing  ;  ice  is. 
therefore,  a  double  refracting  substance. 
Double  refraction  is  displayed  in  a  particu- 
larly impressive  manner  by  Iceland  spar, 
which  is  crystallized  carbonate  of  lime.  The 
difference  of  ethereal  density  in  two  direc- 
tions in  this  crystal  is  very  great,  the  separa- 

I  tion  oi  the  beam  into  the  two  halves   being, 

•  therefore,  particularly  striking. 

Before  you  is  now  projected  an  image  of  our 
carbon -points.  Introducing  the  spar,  the  beam 
which  builds  the  image  is  permitted  to  pass 

j  through  it;  instantly  you  have  the  single  image 

1  divided  into  two.  Projecting  an  image  of  the 
aperature  through  which  the  light  issues  from 
the  electric  lamp,  and  introducing  the  spar, 
two  luminous  disks,  insteadof  one,  appear 
immediately  upon  the  screen.  (See  Fig.  9.,. 


FIG.  9. 


The  two  elements  of  rapidity  of  propaga- 
tion, both  of  sound  and  light,  in  any  sub- 
stance whatever,  are  elasticity  and  density, 
and  the  enormous  velocity  of  light  is  attain- 
able because  the  ether  is  at  the  same  time  of 
infinitesimal  density  and  of  enormous  elas- 
ticity. It  surrounds  the  atoms  of  all  bodies, 
but  seems  to  be  so  acted  upon  by  them  that 
its  density  is  increased  without  a  proportionate 
increase  of  elasticity  ;  this  would  account  for 
the  diminished  velocity  of  light  in  refracting  j 
bodies.  In  virtue  of  the  crystalline  archi-  ; 
tecture  that  we  have  been  considering,  the  j 
ether  in  many  crystals  possesses  different  ! 
densities  in  different  directions  ;  and  the  con- 
sequence is,  that  some  of  these  media  trans- 
mit light  with  two  different  velocities.  Now, 
refraction  depends  wholly  upon  the  change 
of  velocity  on  entering  the  refracting  medium  ; 
and  is  greatest  where  the  change  of  volicity 
is  greatest.  Hence,  as,  in  many  crystals,  we 
have  two  different  velocities,  we  have  also 
two  different  refractions,  a  beam  of  light  being 
divided  by  such  crystals  into  two.  This  ef- 
fect is  called  double  refraction. 


The  two  beams  into  which  the  spar  divides 
the  single  incident-beam  do  not  behave  alike. 
One  of  them  obeys  the  ordinary  law  of  re- 
fraction discovered  by  Snell,  and  this  is 
called  the  ordinary  ray.  The  other  does  not 
obey  the  ordinary  law.  Its  index  of  refrac- 
tion, for  example,  is  not  constant,  nor  do  the 
incident  and  refracted  rays  always  lie  in  the 
same  plane.  It  is,  therefore,  called  the  ex- 
traordinary ray.  Pour  water  and  bisulphide 
of  carbon  into  two  cups  of  the  same  depth  ; 
looked  at  through  the  liquid,  the  cup  that  con- 
tains the  more  strongly-refracting  liquid  will 
appear  shallower  than  the  other.  Place  a 
piece  cf  Iceland  spar  over  a  dot  of  ink  ;  two 
dots  are  seen,  but  one  appears  nearer  than  the 
other.  The  nearest  dot  belongs  to  the  mosr 
strongly-refracted  ray,  which  in  this  case  is 
ths  ordinary  ray.  Turn  the  spar  round,  and 
the  extraordinary  image  of  the  spot  rotates 
round  the  ordinary  one. 

The  double  refraction  of  Iceland  spar  was 
first  treated  in  a  work  published  by  Erasmus 
Bartholitnus,  in  1669.  The  celebrated  Huy- 
ghens  sought  to  account  for  the  phenomenon 


SIX  LECTURES  ON  LIGHT. 


on  the  principles  of  the  wave  theory,  and  he  !  of   the    light   being  the  consequence.     The 


succeeded  in  doing  so.  He  made  highly  im- 
portant observations  on  the  distinctive  charac- 
ter of  the  two  beams  transmitted  by  the  spar. 
Newton,  reflecting  on  the  observations  of 
I  luyghens,  came  to  the  conclusion  that  each 
of  the  beams  had  two  sides  ;  and  from  the 
analogy  of  this  two  sidedness  with  the  two 
<->idedness  of  a  magnet,  wherein  consists 'its 
polarity,  the  two  beams  came  subsequently  to 
be  described  as  polarized. 

We  shall  study  this  subject  of  the  polariza- 
tion of  light  with  great  ease  and  profit  by 
means  of  a  crystal  of  tourmaline.  But  let  us 
start  with  a  clear  conception  of  an  ordinary 
beam  of  light.  It  has  been  already  explained 
that  the  vibrations  of  the  individual  ether- 
particles  are  executed  across  the  line  of  prop- 
agation. In  the  case  of  ordinary  light  we 
are  to  figure  the  ether  particles  as  vibrating 
in  all  directions,  or  azimuths,  as  it  is  some- 
times expressed,  across  this  line. 

Now,  in  a  plate  of  tourmaline  cut  parallel 
to  the  axis  of  the  crystal,  the  beam  of  incident 
light  is  divided  into  two,  the  one  vibrating 
parallel  to  the  axis  ot  the  crystal,  the  other  at 
right  angles  to  the  axis.  'The  grouping  of 
*he  molecules,  and  of  the  ether  associated 
wiih  the  molecules,  reduces  all  the  vibrations 
incident  upon  the  crystal  to  these  two  direc- 
tions. One  of  these  beams,  namely  that  one 
\vhose  vibrations  are  perpendicular  to  the 
c-ixis,  is  quenched  with  exceeding  rapidity  by 
the  tourma'ine,  so  '.hat,  after  having  passed 
thiough  a  very  small  thickness  of  the  crystal, 
the  light  emerges  with  all  its  vibrations  re- 
duced to  a  single  plane.  In  this  condition  it 
is  what  we  call  a  beam  of  plane  polarized 
light. 

A  moment's  reflection  will  show,  if  what 
has  been  stated  be  correct,  that,  on  placing 
a  second  plate  of  tourmaline  with  its  axis 
parallel  to  the  first,  the  light  will  pass  through 
both  ;  but  that,  if  the  axes  be  crossed,  the 


light  that  passes  through  the  one  plate  will 
be  quenched  by  the  other,  a  total  interception 


image  of  a  plate  of  tourmaline,  /  t  (Fig.  10), 
is  now  before  you.  I  place  parallel  to  it 
anot'.er  plate,  tf  tf :  the  green  of  the  crystal 
is  a  little  deepened,  nothing  more.  By  means 
of  an  endless  screw,  I  now  turn  one  of  the 
crystals  gradually  round  ;  as  long  as  the  two 
plates  are  oblique  to  each  other,  a  certain 
portion  of  light  gets  through  ;  but,  when  they 
are  at  right  angles  to  each  other,  the  space 
common  to  both  is  a  space  of  darkness,  as 
shown  in  Fig.  n. 

Let  us  return  to  a  single  plate  ;  and  let  me 
say  that  it  is  on  the  green  light  transmitted 
by  the  tourmaline  that  you  are  to  fix  your  at- 
tention. We  have  now  to  illustrate  the  two- 
sidedness  of  that  green  light.  The  light  sur- 
rounding the  green  image  being  ordinary 
light,  is  reflected  by  a  plane  glass  mirror  in 
all  directions  ;  the  green  light,  on  the  con- 
trary, is  not  so  rcflecteJ.  The  image  of  the 
tourmaline  is  row  horizontal  ;  reflected  up- 
wards, it  is  still  green  ;  reflected  sideways, 
the  image  is  reduced  to  blackness,  because  of 
the  incompetency  of  the  green  1  ght  to  be  re- 
flected in  this  direction.  Making  the  plate 
of  tourmaline  vertical  and  reflecting  it  as 
before,  in  the  upper  image  the  light  is 
quenched  ;  in  the  side  image  you  have  now 
the  green.  Picture  the  thing  clearly.  In 
the  one  case  the  mirror  receives  the  impact 
ot  the  edges  of  the  waves,  ana  the  green  light 
is  quenched.  In  the  other  case  the  sides  of 
the  waves  strike  the  mirror,  and  t  e  green 
light  is  reflected.  To  render  the  extinction 
complete,  the  light  must  be  received  upon 
the  mirror  at  a  special  angle.  What  this 
an^le  is  we  shall  learn  presently. 

The  quality  of  two-sidedness  conferred 
upon  light  by  crystals  may  also  be  conferred 
upon  it  by  ordinary  reflection.  Malus  made 
this  discovery  in  1808,  while  looking  through 
Iceland  spar  at  the  light  of  the  sun  reflected 
from  the  windows  of  the  Luxembourg  palace 
in  I'aris.  I  receive  upon  a  plate  of  window- 
glass  the  beam  f  i  om  our  lamp  ;  a  great  por- 
tion of  the  light  reflected  from  the  glass  is 
polarized  ;  the  vibrations  of  this  reflected 
beam  are  executed,  for  the  most  part,  paral- 
lel to  the  surface  of  the  glass,  and,  if  the 
glass  be  held  so  that  the  beam  shall  make  an 
angle  of  58°  with  the  perpendicular  to  the 
glass,  the  whole  ok.  the  reflected  beam  is  polar- 
ized. It  was  at  this  angle  that  the  image 
of  the  tourmaline  was  completely  quenched 
in  our  former  experiments.  It  is  called  the 
polarizing  angle. 

And  now  let  us  try  to  make  substantially 
the  experiment  of  Malus.  I  receive  the  beam 
from  the  lamp  upon  this  plate  of  glass  and 
reflect  it  through  the  spar.  Instead  of  two 
images,  you  see  but  one.  So  that  the  light, 
when  polarized,  as  it  now  is,  can  only  get 
through  the  spar  in  one  direction,  and  conse- 
[uently  produce  <but  one  image.  Why  is 
this?  In  the  Iceland  spar,  as  in  the  tour- 
maline, all  the  vibrations  of  the  ordinary  light 


SIX  LECTURES  ON  LIGHT. 


are  reduced  to  two  planes  at  right  angles  to 
each  other ;  but,  unlike  the  tourmaline,  both 
beams  are  transmitted  with  equal  facility  by 
the  spar.  The  two  beams,  in  short,  emerg- 
ent from  the  spar  are  polarized,  their  direc- 
tions of  vibration  being  at  right  angles  to 
each  other.  "When,  therefore,  the  light  was 
polarized  by  reflection,  the  direction  of  vibra- 
tion in  the  spar  which  corresponded  to  the 


conclude?  That  the  green  light  will  be 
transmitted  along  the  latter,  which  is  parallel 
to  the  tourmaline,  and  not  along  the  former, 
which  is  perpendicular  to  it.  Hence  we  n  ay 
infer  that  one  image  of  the  tourmaline  will 
show  the  ordinary  green  light  of  the  crystal, 
while  the  other  image  will  be  black.  Let  us 
test  our  reasoning  by  experiment  :  it  is  veri- 
fied to  the  letter.  (Fig.  12.) 


FIG. 

direction  of  vibration  of  the  polarized  beam  | 
transmitted  it,  and  that  direction  only.      But  j 
one  image,  therefore,  was  possible  under  the 
conditions. 

And  now  you  have  it  in  your  power  to 
check  many  of  my  statements,  and  you  will 
observe  that  ^uch  logic  as  connect  our  experi- 
ments is  simply  a  transcript  of  the  logic  of 
Nature.  On  the  screen  before  you  are  the 


Let  us  push  our  test  still  further.  By 
means  of  an  endless  screw,  the  crystal  can 
be  turned  ninety  degrees  round.  The  black 
image,  as  I  turn,  becomes  gradually  brighter 
and  the  bright  one  gradually  darker;  at  an 
angle  of  forty-five  degrees  both  images  are 
equally  bright  (Fig.  13);  while,  when  ninety 
degrees  have  been  obtained,  the  axis  of  the 
crystal  being  then  vertical,  the  bright  and 


two  disks  of  light  produced  by  the  double  re- 
fraction of  the  spar.  They  are,  as  you 
know,  two  images  of  the  aperture  through 
which  the  light  issues  from  the  camera. 
Placing  the  tourmaline  in  front  of  the  aper- 
ture, two  images  of  the  crystal  will  be  ob- 
tained ;  but  now  let  us  reason  out  what  is  to 
be  expected  from  this  experiment.  The  light 
emergent  from  the  tourmaline  is  polarized. 


black  images    have    changed   places.     ('Fig. 
14.) 

Given  two  beams  transmitted  through  Ice- 
land spar,  it  is  perfectly  manifest  that  we 
have  it  in  our  power  to  determine  instantly, 
by  means  of  a  plate  of  tourmaline,  the  direc- 
tions in  which  the  ether-particles  vibrate  in 
the  two  beams.  I  might  place  the  double- 
refracting  spar  in  any  position  whatever.  A 


FIG.  14. 


Placing  the  crystal  with  its  axis  horizontal, 
the  vibrations  of  the  transmitted  light  will  be 
horizontal.  Now  the  spar,  as  already  stated, 
has  two  perpendicular  directions  ot  vibration, 
one  of  which,  at  the  present  moment,  is  ver- 
tical, the  other  horizontal.  What  are  we  to 


minute's  trial  with  the  tourmaline  would 
enable  you  to  determine  the  position  which 
yields  a  black  and  a  bright  image,  and  from 
these  you  would  at  once  infer  the  directions 
of  vih'ntion. 

lurther,    the  two   beams  from  the    spar 


SIX  LECTURES  ON  LIGHT. 


27 


being  thus  polarized,  if  they  be  suitably  re- 
ceived upon  a  plate  of  glass  at  the  polarizing 
angle,  one  of  them  will  be  reflected,  the 
other  not.  This  is  the  conclusion  of  reason 
from  our  previous  knowledge;  but  you  ob- 
serve that  reason  is  justified  by  experiment. 
(Figs.  15  and  16.) 

I  have  said  that  the  whole  of  the  beam  re- 
flected from  glass  at  the  polarizing  angle  is 
polarized;  a  word  must  now  be  addfd  regard- 
ing the  larger  portion  of  the  li^ht  transmitted 
by  the  glass.  The  transmitted  beam  contains 
n  ruantity  of  polarized  light  equal  to  that  of 
tl  e  reflected  beam;  but  this  quantity  is  only 
a  L  action  of  the  whole  transmitted  light.  By 
taking  two  plates  of  glass  instead  of  one,  we 


(B  is  the  birefracting  spar,  dividing  the  incident 
liafnt  into  the  two  beams,  o  and  e.  G  is  the  mirror). 
T. i e  beam  is  here  reflected  laterally.  When  the  re- 
nVction  is  upwards^  the  other  beam  is  reflected,  as 
shown  in  Fig.  16. 


augment  the  quantity  of  the  transmitted  polar- 
ized light;  and, by  taking  a  bundle  of  plates,  we 


so  increase  the  quantity  as  to  render  the  trans- 
mitted beam,  for  all  practical  pu r poses, per- 
fectly polarized.  Indeed,  bundles  of  glass 
plates  are  often  employed  as  a  means  of  fur- 
nishing polarized  light. 

One  word  more.  When  the  tourmalines 
are  crossed,  the  space  where  they  cross  each 
other  is  black.  But  we  have  seen  that  the 
least  obliquity  on  the  part  of  the  crystals  per- 
mits light  to  get  through  both.  Now  sup- 
i  ose,  when  the  two  plates  are  crossed,  that 
we  interpose  a  third  plate  of  tourmaline  be- 
tween them,  wilh  its  axis  obliqus  to  both.  A 
portion  of  the  light  transmitted  by  the  first 
plate  will  get  through  this  intermediate  one. 
But,  after  it  has  got  through,  its  plane  of  vi- 
bration is  changed:  it  is  no  longer  perpendicu- 
lar to  the  axis  cf  the  crystal  in  front.  Hence 
it  will  get  thiough  that  crystal.  Thus,  by 
reasoning,  we  infer  that  the  interposition  of  a 
third  plate  of  tourmaline  will  in  part  abolish 
the  darkness  produced  by  the  perpendicular 
crossing  of  the  other  two  plates.  I  have  not 
a  third  plate  of  tourmaline  ;  but  the  talc  or 
mica  which  you  employ  in  your  stoves  is  a 
more  convenient  substance,  which  acts  in  the 
same  way.  Between  tne  crossed  tourmalines 
I  introduce  a  film  of  this  crystal.  You  see 
the  edge  of  the  film  slowly  descending,  and 
as  it  descends  between  the  tourmalines,  light 
takes  the  place  of  darkness.  The  darkness, 
in  fact,  se«.med  scraped  away  as  if  it  were 
something  material.  This  effect  has  been 
called — and  improperly  called — depolarization. 


LECTURE   IV. 

Chromatic  Phenomena  produced  by  Crystals  on  Polar- 
ized Light:  The  Nicoi  Prism:  Polarizer  and  Ana- 
lyzer :  Action  of  thick  and  thin  Plates  of  Salenite  : 
Colors  dependen.  on  Thickness:  Resolution  of  Po- 
larized Beam  into  two  others  by  the  Selenite  :  One 
of  them  mure  retarded  than  the  other:  Recom- 
pounding  of  the  two  Systems  of  Waves  by  the  Ana- 
lyzer: Interference  thus  rendered  possible  :  Conse- 
quent Production  of  Colors:  Action  of  Bodies 
Mechanically  strained  or  pressed  :  Action  of  Sono- 
rous Vibrations :  Action  of  Glass  strained  or  pressed 
by  Heat:  Circular  Polarization:  Chromatic  Phe- 
nomena produced  by  Quartz :  The  Magnetization 
of  Light :  Rings  surrounding  the  Axes  of  Crystals : 
Biaxal  and  Uniaxal  Crystals :  Grasp  of  the  Undu- 
latory  Theory. 

We  now  stand  upon  the  threshold  of  a  new 
and 'splendid  optical  domain.  We  have  to 
examine,  this  evening,  the  chromatic  phe- 
nomena produced  by  the  action  ot. crystals, 
and  double-refracting  bodies  gene:  ally,  upon 
polarized  light.  For  along  time  investigators 
were  compelled  to  employ  plates  of  tourmaline 
for  this  purpose,  and  the  progress  they  made 
with  so  defective  a  means  of  inquiry  is  aston- 
ishing. But  these  men  had  their  hearts  in 
their  work,  and  were  on  this  account  enabled 
to  extract  great  results  from  small  instrumen- 
tal appliances.  But  we  have  better  apparatus 
now.  You  have  seen  the  two  beams  emer- 
gent from  Iceland  spar,  and  have  proved 
them  to  be  polarized.  If  we  could  abolish 


28 


SIX  LECTURES  ON  LIGHT. 


cne  of  these  beams,  vre  might  employ  the 
other  for  experiments  on  polarized  light. 

These  beams,  as  you  know,  are  refracted 
differently,  and  from  this  we  are  able  to  infer 
that  under  some  circumstances  the  one  may 
be  totally  reflected,  and  the  other  not  An 
optician,  named  Nicol,  cut  a  crystal  of  Ice- 
land spar  in  two  in  a  certain  direction.  He 
polished  the  severed  urfaces,  and  reunited 
them  by  Canada  balsam,  the  surface  of  union 
being  so  inclined  to  the  beam  traversing  the 
spar  that  the  ordinary  ray,  which  is  the  most 
highly  refracted,  was  totally  reflected  by  the 
balsam,  while  the  extraordinary  ray  was  per- 
mitted to  pass  on.  The  invention  of  the 
Nicol  prism  was  a  great  step  in  practical  op- 
tics, and  quite  recently  such  prisms  have 
been  constructed  of  a  size  which  enables 
audiences  like  the  present  to  witness  the 
chromatic  phenomena  of  polarized  light  to  a 
degree  altogether  unattainable  a  short  time 
ago.  The  two  prisms  here  before  you  belong 
to  my  excellent  friend,  Mr.  William  Spottis- 
woode,  and  they  were  manufactured  by  Mr. 
Ladd.  I  have  with  me  another  pair  of  very 
noble  prisms,  still  larger  than  these,  manu- 
factured for  me  by  Mr.  Browning,  who  has 
gained  so  high  and  well-merited  a  reputation 
in  the  construction  of  spectroscopes. 

These  two  Nicol  prisms  play  the  same 
part  as  the  crystals  of  tourmaline.  Placed 
with  their  directions  of  vibration  parallel, 
the  light  passes  through  both.  When  these 
directions  are  crossed,  the  light  is  quenched. 
Introducing  a  film  of  mica  between  the 
prisms,  the  light  is  in  pirt  restored.  But 
notice,  when  t»he  film  of  mica  is  thin,  you 
have  sometimes  not  only  light,  but  colored 
light.  Our  work  for  some  time  to  come  will 
be  the  examination  of  th^se  colors.  With 
this  view,  I  will  take  a  representative  crystal, 
one  easily  dealt  with;  the  crystal  gypsum,  or 
selenite,  which  is  crystallized  sulphate  of 
lime.  Between  the  crossed  Nicols  I  place  a 
thick  plate  of  this  crystal;  like  the  mica,  it 
restores  the  light,  but  it  produce^  no  color. 
With  my  penknife  I  take  a  thin  splinter  from  transmitted  vibration. 


heart's-ease,  the  colors  of  which  you  might 
safely  defy  the  artist  to  reproduce.  By  turn- 
ing the  front  Nicol  ninety  degrees  round,  we 
pass  through  a  colorless  phase  to  a  series  of 
colors  complementary  to  the  lorner  ones. 
Here,  for  example,  is  a  rose  tree  with  red 
flowers  and  green  leaves;  turning  the  prism 
ninety  degrees  round,  we  obtain  a  green 
flower  and  red  leaves.  All  these  wonderful 
chromatic  effects  have  definite  mechanical 
causes  in  the  motions  of  the  ether.  The 
principle  of  interference,  duly  applied  and 
interpreted,  explains  them  all. 

By  this  time  you  have  learned  that  the 
word  "  light"  may  be  used  in  two  deferent 
senses  ;  it  may  mean  the  impression  made 
upon  consciousness,  or  it  may  mean  the  phys- 
ical agent  which  makes  the  impression.  It 
is  with  the  agent  that  we  have  to  occupy  our- 
selves at  present.  That  agent  is  the  motion 
of  a  substance  which  fills  all  space,  and  sur- 
rounds the  atoms  and  molecules  of  bodies. 
To  this  interstellar  and  interatomic  medium 
definite  mechanical  properties  are  ascribed, 
and  we  deal  with  it  as  a  body  possessed  of 
these  p  operties.  In  mechanics  we  have  the  . 
composition  and  resolution  of  forces,  and  of 
motions,  extending  to  the  composition  and 
resolution  of  -vibrations.  We  treat  the  lumi- 
niferous  ether  on  mechanical  principles,  and 
from  the  composition,  resolution,  and  inter- 
ference of  its  vibrations,  we  deduce  all  the 
phenomena  displayed  by  crystals  in  polarized 
light. 

Let  us  take,  as  an  example,  the  crystal  of 
tourmaline,  with  which  we  are  now  so  famil- 
iar. Let  a  vibration  cross  thiscrysta*  oblique 
to  its  axis  ;  we  have  seen  by  ex^r  ^riuunc  Mat 
a  portion  of  the  light  will  pa-.  '>  through. 
How  much,  we  determine  in  this  way :  Draw 
a  straight  line  representing  the  intensity  of 
the  vibration  before  it  reaches  the  tourmaline, 
and  from  the  two  ends  of  this  line  draw  two 
perpendiculars  to  the  axis  of  the  crystal ;  the 
distance  between  the  feet  of  these  two  per- 
pendiculars will  represent  the  intensity  of  tne 


this  crystal  and  place  it  between  the  pri> 
its  image  on  the  screen  glows  with  thericuebi 
colors.  Turning  the  prism  in  front,  these 
colors  gradually  fade,  disappear,  but  by  con- 
tinuing the  rotation  until  the  vibrating  sec- 
tions of  the  prisms  are  parallel,  vivid  colors 
again  appear,  but  these  colors  are  comple- 
mentary to  the  former  ones. 

Some  patches  of  the  splinter  appear  of  one 
color,  some  of  another.  These  differences 
ar,;  due  to  the  different  thicknesses  of  the 
film.  If  the  thickness  be  uniform,  the  color 
is  uniform.  Here,  for  instance,  is  a  stellar 
shape,  every  lozenge  of  the  star  being  a  film 
of  gypsum  of  uniform  thickness.  Each 
lozenge,  you  observe,  shows  a  brilliant  uni- 
form color.  It  is  easy,  by  shaping  our  films 
so  as  to  represent  flowers  or  other  objects, 


Follow  me  now  while  I  endeavor  to  make 


clear  to  you    what   occurs   when   a 


of 


gypsum  is  placed  between  the  Nicol  prisms. 
But,  at  the  outset,  let  us  establish  still 
further  the  analogy  between  the  action  of  the 
prisms  and  that  of  two  plates  of  tourmaline, 
The  plates  are  now  crossed,  and  you  see  that 
by  turning  the  film  round,  it  may  be  placed 
in  a  position  where  i*"  has  no  power  to  Abolish 
the  darkness.  Why  is  this?  The  answer  is 
that  in  the  gypsum  there  are  two  directions, 
at  right  angles  to  each  other,  which  the  wave* 
of  light  aie  constrained  to  follow,  and  that 
now  one  of  these  directions  is  parallel  to  one 
of  the  axes  of  the  tourmaline,  and  the  other 
parallel  to  the  other  axis.  When  this  is  the 
case,  the  film  exercises  no  sensible  aciion 
upon  the  light.  But  now  I  turn  the  fil.n  so 


to  exhibit  such  objects  i.i  colors  unattainable    as  to  render  its  direction  of  vibration  oblique 
by  art.      Hei-e,  for  example,  is  a  specimen  of  >  to  the  axes  ;    then  you  see  it  has  the  power, 


SIX  LECTURES  ON  LIGHT. 


demonstrated  in  tne  last  lecture,  of  restoring 
the  light. 

Let  us  now  mount  our  Nicol  prisms  and 
cross  them  as  we  crossed  the  tourmalines. 
Introducing  our  film  of  gypsum  between  them, 
you  notice  tnat  in  one  particular  position  the 
film  has  no  power  whatever  over  the  field  of 
view.  But,  when  the  film  is  turned  a  little 
way  round,  the  light  passes.  We  have  now 
to  understand  the  mechanism  by  which  this 
is  effected. 


Firstly,  then,  we  have  this  first  prism  which 
receives  the  light  emergent  from  the  electric 
lamp,  and  which  is  called  the  polarizer.  Then 
we  have  the  p  ate  of  gypsum,  placed  at  S 
(Fig.  17),  and  then  the  prism  in  front,  which 
is  called  the  analyzer.  On  its  emergence 
from  the  first  prism,  the  light  is  polarized  ; 
and  in  the  particular  case  now  before  us,  its 
vibrations  are  executed  in  an  horizontal  plane. 
The  two  directions  of  vibration  of  '.he  gypsum, 
placed  at  S,  are  now  oblique  to  the  horizon. 
Draw  a  rectangular  cross  upon  paper  to  rep- 
resent the  two  directions  of  vibration  within 
the  gypsum.  Draw  an  oblique  line  to  repre- 
sent the  intensity  of  the  vibration  when  it 
reaches  the  gvpsum.  Let  fall  from  the  two 
ends  of  this  line  two  perpendiculars  on  each 
of  the  arms  of  the  cross  ;  then  the  distances 
between  the  feet  of  these  perpendiculars  rep- 
resent the  intensities  of  two  rectangular  vi- 
brations which  are  the  equivalents  of  the  first 
single  vibration.  Thus  the  polarized  ray, 
when  it  enters  the  gypsum,  is  resolved  into 
two  others,  vibrating  at  right  angles  to  each 
other. 

Now,  in  one  of  those  directions  of  vibration 
the  ether  is  more  sluggish  than  in  the  other  ; 
and,  as  a  consequence,  the  waves  that  follow 
this  direction  are  more  retarded  than  the 
others.  The  waves  of  both  systems,  in  fact, 
are  shortened  when  they  enter  the  gypsum, 
but  the  one  system  is  more  shortened  than  the 


other.  You  can  readily  imagine  that  in  this 
way  the  one  system  of  waves  may  get  half  a 
wave-length,  or  indeed  any  number  of  half 
wave-lengths,  in  advance  of  the  other.  The 
possibility  of  interference  here  flashes  upon 
the  mind.  A  little  consideration,  however, 
renders  it  evident  that,  as  long  as  the  vibra- 
tions are  executed  at  right  angles  to  each 
other,  they  cannot  quench  each  other,  no 
matter  what  the  retardation  may  be.  This 
brings  us  at  once  to  the  part  played  by  the 
analyzer.  Its  sole  function  is  to  recompound 
the  two  vibrations  emergent  from  the  gypsum. 
It  reduces  them  to  a  single,  plane,  where,  if 
one  of  them  be  retarded  by  the  proper 
amount,  extinction  can  occur.  But  here,  as 
in  the  case  of  thin  films,  the  different  lengths 
of  the  waves  of  light  come  into  play.  Red 
will  require  a  greater  thickness  to  produce  the 
retardation  necessary  for  extinction  than  blue; 
consequently,  when  the  longer  waves  have 
been  withdrawn  by  interference,  the  shorter 
ones  remain  and  confer  their  colors  on  the 
film  of  gypsum.  Conversely,  when  the 
shorter  waves  have  been  withdrawn,  the 
thickness  is  such  that  the  longer  waves  re- 
main. An  elementary  consideration  suffices 
to  show  that,  when  the  directions  of  vibration 
of  prisms  and  gypsum  enclose  an  angle  of 
forty-five  degrees,  the  colors  are  at  their 
maximum  brilliancy.  When  the  film  is 
i  urned  from  this  direction,  the  colors  gradu- 
ally fade,  until,  at  the  point  where  the  direc- 
tions are  parallel,  they  disappear  altogether. 

A  knowledge  of  these  phenomena  1'i  best 
obtained  by  means  of  a  model  of  wood  or 
pasteboard  representing  the  plate  of  gypsum, 
ts  planes  of  vibration,  and  also  those  of  the 
polarizer  and  analyzer.  On  these  planes  the 
waves  may  be  drawn,  showing  the  resolution 
of  the  first  polarized  ray  into  two  others,  and 
then  the  reduction  of  tne  two  vibrations  to  a 
common  plane.  Following  out  rigidly  the 
nte.-action  of  the  two  systems  of  waves,  we 
are  taught  by  such  a  model  that  a'l  the  phe- 
nomena of  color,  obtained  when  the  planes  of 
vibration  of  the  two  Nicols  are  parallel,  are 
displaced  by  the  complementary  phenomena 
when  the  Nicols  are  perpendicular  to  each 
other. 

In  considering  the  next  point,  for  the  sake 
of  simplicity,  we  will  operate  with  monochro- 
matic light — with  red  light,  for  example. 
Supposing  that  a  certain  thickness  of  the  gyp- 
sum produces  a  retardation  of  half  a  wavo- 
"ength,  twice  this  thickness  will  produce  a 
retardation  of  two  half  wave-lengths ;  three 
times  this  thickness  a  retardation  of  three 
half  wave-lengths,  and  so  on.  Now,  when 
the  Nicols  are  parallel,  the  retardatioi  of 
half  a  wave-length,  or  of  any  odd  number  of 
half  wave-lengths,  produces  extinction:  at  ail 
thicknesses,  on  the  other  hand,  which  corre- 
spond to  a  retardation  of  an  even  number  of 
half  wave-lengths,  the  two  beams  support  each 
other,  when  they  are  brought  to  a  common 
plane  by  the  analyzer.  S  pposing,  then, 


30 


SIX  LECTURES  ON  LIGHT. 


that  we  take  a  plate  of  a  wedge-form,  which 
grows  gradually  thicker  from  edge  to  back, 
we  ought  to  expect  in  red  light  a  series  of 
recurrent  bands  of  light  and  darkness  ;  the 
dark  bands  occurring  at  thicknesses  which 
produce  retardations  of  one,  three,  five,  etc. , 
half  wave-lengths,  while  the  light  bands  occur 
between  the  dark  ones.  Experiment  proves 
the  wedge-shaped  crystal  to  show  these  bands; 
but  they  are  far  better  shown  by  this  circular 
film,  which  is  so  worked  as  to  be  thinnest  at 
the  centre,  gradually  increasing  in  thickness 
from  the  centre  outwards.  These  splendid 
rings  of  light  and  darkness  are  thus  produced. 

When,  instead  of  employing  red  light,  we 
employ  blue,  the  rings  are  also  seen  ;  but,  as 
they  occur  at  thinner  portions  of  the  film, 
they  are  smaller  than  the  rings  obtained  with 
the  red  light.  The  consequence  of  employ- 
ing white  light  may  now  be  inferred  :  inas- 
much as  the  red  and  the  blue  fall  in  different 
places,  we  have  iris-colored  rings  produced  by 
the  white  light. 

Some  of  the  cl^omatic  effects  of  irregular 
crystallization  are  beautiful  in  the  extreme. 
Could  I  introduce  between  our  Nicols  a  pane 
of  glass  covered  by  those  frost-ferns  which 
the  cold  weather  renders  now  so  frequent, 
rich  colors  would  be  the  result.  The  beauti- 
ful effects  of  irregular  crystallization  on  glass 
plates,  now  presented  to  you,  illustrate  what 
you  might  expect  from  the  frosted  window- 
pane.  And  not  only  do  crystalline  bodies 
act  thus  upon  light,  but  almost  all  bodies  that 
possess  a  definite  structure  do  the  same.  As 
a  general  rule,  organic  bodies  act  in  this  way; 
for  their  architecture  implies  an  arrangement 
of  the  ether  which  involves  double  refraction. 
A  film  of  horn,  or  the  section  of  a  shell,  for 
example,  yields  very  beautiful  colors  in  polar- 
ized light.  In  a  tree,  the  ether  certainly  pos- 
sesses different  degrees  of  elasticity  along  and 
across  the  fibre;  and,  were  wood  transparent, 
this  peculiarity  of  molecular  structure  would 
infallibly  reveal  itself  by  chromatic  phe- 
nomena like  those  that  you  have  seen.  But 
not  only  do  bodies  built  permanently  by 
Nature  behave  in  this  way,  but  it  is  possible, 
as  shown  by  Brewster,  to  confer,  by  strain  or 
by  pressure,  a  temporary  double-refracting 
structure  upon  non-crystalline  bodies,  such  as 
common  glass. 

When  I  place  this  bar  of  wood  across  my 
knee  and  seek  to  break  it,  what  is  the 
mechanical  condition  of  the  bar  ?  It  bends, 
and  its  convex  surface  is  strained  longitudi- 
nally; its  concave  surface,  that  next  my  knee, 
is  longitudinally  pressed.  Both  in  the 
strained  portion  and  in  the  pressed  portion 
the  ether  is  thrown  into  a  condition  which 
would  rend  r  the  wood,  were  it  transparent, 
double  refracting.  Let  us  repeat  the  experi- 
ment with  a  bar  of  glass.  Between  the 
crossed  Nicols  I  introduce  such  a  bar.  By 
ihe  dim  residue  of  light  lingering  upon  the 
screen,  you  see  the  imjge  of  the  glass,  but  it 
has  no  effect  'ipon  the  light.  I  simply  bend 


the  gla-s  bar  with  my  finger  and  thumb, 
keeping  its  length  oblique  to  the  directions  of 
vibration  in  the  Nicols.  Instantly  light 
flashes  out  upon  the  screen.  The  two  sides 
of  the  bar  are  illuminated,  the  ed^es  most, 
for  here  the  strain  and  pressure  are  greatest. 
In  passing  from  strain  to  pressure,  we  cross  a 
portion  of  the  glass  where  neither  is  exerted. 
This  is  the  so-called  neutral  axis  of  the  bar 
of  glass,  and  along  it  you  see  a  dark  band, 
indicating  that  the  glass  along  this  axis  exer- 
cises no  c  ction  upon  the  light.  By  employ- 
ing the  force  of  a  press,  instead  of  the  force 
of  my  finger  and  thumb,  the  brilliancy  of  the 
light  is  greatly  augmented. 

Again,  I  have  here  a  square  of  glass  which 
can  be  inserted  into  a  press  of  another  kind. 
Introducing  the  square  between  the  prisms, 
its  neutrality  is  declared  ;  but  it  can  hardly 
be  held  sufficiently  loosely  to  prevent  its 
action  from  manifesting  itself.  Already, 
though  the  pressure  is  infinitesimal,  you  see 
spots  of  light  at  the  points  where  the  press  is 
in  contact  with  the  glass.  I  now  turn  this 
screw.  Instantly  the  image  of  the  square  of 
glass  flashes  out  upon  the  screen.  You  see 
luminous  spaces  separated  from  each  other 
by  dark  bands.  Every  pair  of  adjacent 
luminous  spaces  is  in  opposite  mechanical 
conditions.  On  one  side  of  the  dark  band 
we  have  strain,  on  the  other  side  pressure ; 
while  the  dark  band  marks  the  neutral  axis 
between  both.  I  now  tighten  the  vise,  and 
you  see  color ;  tighten  still  more,  and  tha 
colors  appear  as  rich  as  those  presented  by 
crystals.  Releasing  the  vise,  the  colors 
suddenly  vanish  ;  tightening  suddenly,  they 
reappear.  From  the  colors  of  a  soap-bubble 
Newton  was  able  to  infer  the  thickness  of  the 
bubble,  thus  uniting  by  the  bond  of  thought 
apparently  incongruous  things.  From  the 
colors  here  presented  to  you,  the  magnitude 
of  the  pressure  employed  might  be  inferred. 
Indeed,  the  late  M.  Werthtim,  of  Paris,  in- 
vented an  instrument  for  the  determination 
of  strains  and  pressures  by  the  colors  of 
polarized  light,  which  exceeded  m  accuracy 
all  other  instruments  of  the  kind. 

You  know  that  bodies  are  expanded  by 
heat  and  contracted  by  cold.  If  the  heat  be 
applied  with  perfect  uniformity,  no  local 
strains  or  pressures  come  into  play  ;  but,  if 
one  portion  of  a  solid  be  heated  and  others 
not,  the  expansion  of  the  heated  portion  intro- 
duces strains  and  pressures  which  reveal 
themselves  under  the  scrutiny  of  polarized 
light.  When  a  square  of  common  window- 
glass  is  placed  between  the  Nicols,  you  see 
its  dim  outline,  but  it  exerts  no  action  on  the 
polarized  light.  Held  for  a  moment  over  ihe 
flame  of  a  spirit-lamp,  on  reintroducing  it 
between  the  Nicols,  light  flashes  out  upon 
the  screen.  Here,  as  in  the  case  of  mechan- 
ical action,  you  have  spaces  of  strain  divided 
by  neutral  axes  from  spaces  of  pressure. 

Let  us  apply  the  heat  more  symmetrically. 
This  small  square  of  glass  is  perforated  at 


SIX  LECTURES  ON  LIGHT. 


the  rentre,  and  into  the  orifice  a  bit  of  copper 
wire  is  introduced.  Placing  the  square  be- 
tween the  prisms,  and  heating  the  copper, 
the  heat  passes  by  conduction  along  the  wire 
to  the  glass,  throug  i  which  it  spreads  from 
the  centre  outwards.  You  see  a  dim  cross 
bounding  four  luminous  quadrants  growing 
up  and  becoming  gradually  black  by  compari- 
son with  the  adjacent  brightness.  And  as,  in 
the  case  of  pressure,  we  produced  colors,  so 
here  als%  by  the  proper  application  of  heat, 
gorgeous  chromatic  effects  may  be  produced, 
and  they  may  be  rendered  permanent  by  first 
heating  the  glass  sufficiently,  and  then  cool- 
ing it,  so  that  the  chilled  mass  shall  remain 
in  a  state  of  strain  and  pressure.  Two  or 
three  examples  will  illustrate  this  point.  The 
colors,  you  observe,  are  quite  as  rich  as  those 
obtained  in  the  case  of  crystals. 

And  now  we  have  to  push  these  considera- 
tions to  a  final  illustration.  Polarized  light 
may  be  turned  to  account  in  various  ways  as 
an  analyzer  of  molecular  condition.  A  strip 
of  glass  six  feet  long,  two  inches  wide,  and 
a  quarter  of  an  inch  thick,  is  held  at  the 
centre  between  my  finger  and  thumb.  I 
sweep  over  one  of  its  halves  a  wet  woolen 
rag ;  you  hear  an  acute  sound,  due  to  the 
vibrations  of  the  glass.  What  is  the  condi- 
tion of  the  glass  while  the  sound  is  heard  ? 
This  .  its  two  halves  lengthen  and  shorten  in 
quick  succession.  Its  t*vo  ends,  therefore, 
aie  in  a  state  of  quick  vibration  ;  but  at  the 
centre  the  pulses  from  the  two  ends  alter- 
nately meet  and  retreat.  Between  their 
opposing  actions,  the*  glass  at  the  centre  is 
kept  motionless  ;  but,  on  the  other  hand,  it 
is  alternately  strained  and  compressed.  The 
state  of  the  glass  may  be  illustrated  by  a  row 
of  spots  of  light,  as  the  propagation  of  a 
sonorous  pulse  was  illustrated  in  a  former 


m 


FIG.  1 8. 


lecture.  By  a  simple  mechanical  contrivance 
the  spots  are  made  to  vibrate  to  and  fro. 
The  terminal  dots  have  the  largest  amplitude 


of  vibration,  while  those  at  the  centre  are 
alternately  crowded  together  and  torn 
asunder,  the  centre  one  not  moving  at  all. 
The  condition  of  the  sounding  strip  of  glass 
is  here  correct'y  represented.  In  Fig.  18,  A 
B  represents  the  glass  rectangle  with  its 
centre  condensed  ;  while  A'  B'  represents  th« 
same  rectangle  with  its  centre  rarefied. 

If  we  introduce  the  glass  s  s'  (Fig.  19)  be- 
tween  the  crossed  Nicols,  taking  care  to  keep  \ 
the  strip  oblique  to  the  direction  of  vibration 
of  the  Nicols,  and  sweep  our  wet  rubber  over 
the  glass,  this  may  be  expected  to  occur  :  At 
every  moment  of  compression  the  light  wiil 
flash  through  ;  at  every  moment  of  strain  the 
light  will  also  flash  through  ;  and  these  states 
of  strain  and  pressure  will  follow  each  other 
so  rapidly  that  we  may  expect  a  permanent 
luminous  impression  to  be  made  upon  the 
eye.  By  pure  reasoning,  therefore,  we  reach 
the  conclusion  that  the  light  will  be  revived 
whenever  the  glass  is  sounded.  That  it  is  so, 
experiment  testifies :  at  every  sweep  of  the 
rubber,  a  fine  luminous  disk  (o)  flashes  out 
upon  the  screen.  The  experiment  may  be 
varied  in  this  way :  Placing  in  fron  of  the 
polarizer  a  plate  of  unannealed  glass,  you 
have  those  beautiful  colored  rings,  intersected 
by  a  black  cross.  Every  sweep  of  the  rubber 
not  only  abolishes  the  rings,  but  introduces 
complementary  ones,  the  black  cross  being 
for  the  moment  supplanted  by  a  white  one. 
This  is  a  modification  of  an  experiment 
which  we  owe  to  Biot.  His  apparatus,  how- 
ever, confined  the  observation  of  it  to  a  single 
person  at  a  time. 

But  we  have  to  follow  the  ether  still 
further.  Suspended  before  you  is  a  pendu- 
lum, which,  when  drawn  aside  and  then 
liberated,  oscillates  to  and  fro.  If  when  the 
pendulum  is  passing  the  middle  point  of  its 
excursion,  I  impart  a  shock  to  it  tending  to 
drive  it  at  right  angles  to  its  present  course, 
what  occurs  ?  The  two  impulses  compound 
themselves  to  a  vibration  oblique  in  direction 
to  the  former  one,  but  the  pendulum  oscil- 
lates in  a  plane.  But,  if  the  rectangular 
hock  be  imparted  to  the  pendulum  when  it 
is  at  the  limit  of  its  swing,  then  the  com- 
pounding of  the  two  impulses  causes  the  sus- 
pended ball  to  describe  not  a  straight  line, 
but  an  ellipse  ;  and,  if  the  shock  be  compe- 
tent of  itself  to  produce  a  vibration  of  the 
same  amplitude  as  the  first  one,  the  ellipse 
becomes  a  circle.  But  why  do  I  dwell  upon 
these  things  ?  Simply  to  make  known  to  you 
the  resemblance  of  these  gross  mechanical 
vibrations  to  the  vibrations  of  light.  I  'hold 
"n  my  hand  a  plate  of  quartz  cut  from  the 
crystal  perpendicular  to  its  axis.  This  crys- 
tal thus  cut  possesses  the  extraordinary  power 
of  twisting  the  plane  of  vibration  of  a  polar- 
'zed  rav  to  an  extent  dependent  on  the  thick- 
ness of  the  crystal.  And  the  more  refrangi- 
ble the  light  the  greater  is  the  amount  of 
twisting,  so  that,  when  white  light  is  em. 
ployed,  its  constituent  colors  are  thus  drawn 


SIX  LECTURES  ON  LIGHT. 


asunder.  Placing  the  quartz  between  the 
polarizer  and  the  analyzer,  you  see  this 
splendid  color,  and,  turning  the  analyzer  in 
front,  from  right  to  left,  the  other  colors 
apoear  in  succession.  Specimens  of  quartz 
have  been  found  which  require  the  analyzer 
to  be  turned  from  left  to  right,  to  obtain  the 
same  succession  of  colors.  Crystals  of  the 
first  class  are  therefore  called  right-handed, 
and,  of  the  second  class,  left  handed  crystals. 
With  profound  sagacity,  Fresnet,  to  whose 
genius  we  mainly  owe  the  expansion  and 
final  triumph  of  the  undulatory  theory  of 
light,  reproduced  mentally  the  mechanism  of 
these  crystals,  and  showed  their  action  to  be 
due  to  the  circumstance  that,  in  them,  the 
waves  of  ether  so  act  upon  each  other  as  to 
produce  the  condition  represented  by  our 
rotating  pendulum.  Instead  of  being  plane 
polarized,  the  light  in  rock  crystal  is  circu- 
larly polarized.  Two  such  rays  transmitted 
along  the  axis  of  the  crystal,  and  rotating  in 


although  the  mixture  of  blue  and  yellow  pig« 
ments  produces  green,  the  mixture  of  blue 
and  ye.low  lights  produces  white.  By  en- 
larging our  aperture,  the  two  images  pro- 
duced by  the  spar  are  caused  to  approach 
each  other,  and  finally  to  overlap.  The  one 
is  now  a  vivid  yellow,  the  other  a  vivid  blue, 
and  you  notice  that  where  the  colors  arts 
superposed  we  have  a  pure  white.  (See  Fig. 
20,  where  N  is  the  nozzle  of  the  lamp,  Q  the 
quartz  plate,  L  a  lens,  and  B  the  birefracting 
spar.  The  two  images  overlap  at  O,  and 
produce  white  by  their  mixture.) 

This  brings  us  to  a  point  of  our  inquiries 
which,  though  not  capable  of  brilliant  illus- 
tration, is  nevertheless  so  likely  to  affect  pro- 
foundly the  future  course  of  scientific  thought 
that  I  am  unwilling  to  pass  it  over  without 
reference.  I  refer  to  the  experiment  which 
Fa-'aday,  its  discoverer,  called  the  magnetiza- 
tion of  light.  The  arrangement  for  thL 
celebrated  experiment  is  now  before  you. 


FIG.  19. 


opposite  directions,  when  brought  to  inter- 
ference by  the  analyzer,  are  demonstrably 
competent  to  produce  the  observed  phe- 
nomena. 

1  now  abandon  the  analyzer,  and  put  in  its 
place  the  piece  of  Iceland  spar  with  which 
we  have  already  illustrated  double  refraction. 
The  two  images  of  the  carbon-points  are 
now  before  you.  Introducing  a  plate  of 
quartz  between  the  polarizer  and  the  spar, 
the  two  images  glow  with  complementary 
colors.  Employing  the  image  of  an  aperture 
instead  of  that  of  the  carbon-points,  .we  have 
two  complementary  colored  circles.  As  the 
analyzer  is  caused  to  rotate,  the  colors  pass 
through  various  changes  ;  but  they  are  al- 
ways complementary  to  each  other.  If  the 
one  be  red,  the  other  will  be  green  ;  if  the 
one  be  yellow,  the  other  will  be  blue.  Here 
we  have  it  in  our  power  to  demonstrate  afresh 
a  statement  made  in  a  former  lecture,  that,  i 


We  have  first  our  electric  lamp,  then  a  Nice* 
prism,  to  polarize  the  beam  emergent  from 
the  lamp  ;  then  an  electro-magnet,  then  a 
second  Nicol  prism,  and  finally  our  screen, 
At  the  present  moment  the  prisms  are 
crossed,  and  the  screen  is  dark.  I  place 
from  pole  to  pole  of  the  electro-magnet  a 
cylinder  of  a  peculiar  kind  of  glass,  first 
made  by  Faraday,  and  called  Faraday's  heavy 
glass.  Through  this  glass  the  beam  from 
the  polarizer  now  passes,  being  intercepted 
by  the  Nicol  in  front.  I  now  excite  the 
magnet,  and  instantly  light  appears  upon  the 
screen.  On  examination,  we  find  that,  by 
the  action  of  the  magnet  upon  the  ether  con- 
tained within  the  heavy  glass,  the  plane  ot 
vibration  is  caused  to  rotate,  thus  enabling 
the  light  to  get  through  the  analyzer. 

The  two  classes  into  which  quartz-crystals 
are  divided  have  been  already  mentioned. 
In  my  hand  I  hold  a  compound  piate,  one- 


SIX  LKC  I  UKi  h 


LIGHT. 


•*UtJ£  of  it  taken  from  a  right-handed  and  the 
other  from  a  left-handed  crystal.  Placing 
the  plate  in  front  of  the  polarizer,  we  turn 
one  of  the  Nicols  until  the  two  halves  of  the 
plate  show  a  common  puce  color.  This 
yields  an  exceedingly  sensitive  means  of  ren- 
dering the  action  of  a  magr.et  upon  light 
jiaible.  By  turning  either  the  polarizer  or 


:he  analyzer  through  the  smallest  angle,  tne 
uniformity  of  the  col-  r  disappears,  and  the 
two  halves  of  the  quartz  show  different  colors. 
The  magnet  also  produces  this  effect.  The 
puce-colored  circle  is  now  befcre  you  on  the 
screen.  (See  Fig.  21  for  the  arrangement  of 
the  experiment.  N  is  he  nozzle  of  the  lamp, 
H  the  first  ?<ico),  Q  the  biquartz  plate,  L  a 
lens,  M  the  electro-magnet,  and  P  the  second 
Xicol.)  Exciting  the  magnet,  one  half  of 
the  image  becomes  Suddenly  red,  the  other 
half  green.  Interrupting  the  current,  the 
two  colors  fade  away,  and  the  primitive  puce 
is  restored.  The  action,  moreover,  depends 
upon  the  polarity  of  the  magnet,  or,  in  other 
words,  on  the  direction  of  tne  current  which 
surrounds  the  magnet.  Reversing  the  cur- 
rent, the  red  and  green  reappear,  but  they 
have  changed  places  The  red  was  for- 
merly to  the  fight,  and  the  green  to  the  left ; 
the  green  is  now  to  the  right,  and  the  red  to 
the  left.  With  the  most  exquisite  ingenuity, 
Faraday  analyzed  all  those  actions  a.  d stated 
their  laws.  This  experiment,  however,  long 
remained  rather  as  a  scientific  curiosity  than  as 
a  fruitful  germ.  That  it  would  bear  fruit  of 
the  highest  :mportance,  Faraday  felt  pro- 
foundly convinced,  and  recent  jesearches  are 
on  the  way  to  verify  his  conviction: 

A  few  words  more  are   necessary  to  com- 
plete our  knowledge  oi   the    wonderful  inter-  i 
action  between  ponderable  molecules  and  the  ! 
ether  interfused  among  them.     Symmetry  of  i 


molecular  arrangement  implies  symmetry  on 
the  part  of  the  ether  ;  atomic  dissymmetry, 
on  the  other  hand,  involves  the  dissymmetry 
of  the  ether,  and,  as  a  consequence,  double 
refraction.  In  a  certain  class  of  crystals  the 
structure  is  homogeneous,  and  such  crystals 
produce  no  double  refraction.  In  certain 


other  crystals  the  mol  jles  are  ranged  sym- 
metrically around  P  ,ertain  line,  and  not 
around  others.  A'  .g  the  former,  therefore, 
the  ray  is  undiv  .ed,  while  along  ail  the 
others  we  have  double  refraction  Ice  is  a 
familiar  examp'e  ;  it  is  built  with  perfect 
symmetry  around  the  perpendiculars  to  the 
phnes  of  freezing,  and  a  ray  sent  through 
ice  in  this  direction  is  not  doubly  refracted  ; 
whereas,  in  all  o^her  directions,  it  is.  Ice- 
land spar  is  another  example  of  the  same 
kind  :  its  molecules  are  built  symmetrically 
round  the  line  uniting  the  two  blunt  angles 
of  the  rhomb,  in  thf*>  direction  a  ray  suffers 
no  double  refraction,  in  all  others  it  does. 
This  direction  of  dou\  !c  refraction  is  called 
the  optic  axis  oi  the  ci \Vi\l. 

Hence,  if  a  plate  be  oat  from  a  crystal  of 
Iceland  spar  perpendicular  to  the  axis,  all 
rays  sent  across  this  plate  :n  the  dirction  of 
the  axis  will  psoduce  but  cne  image.  But, 
the  moment  we  deviate  from  '.he  parallelism 
with  the  axis,  double  refraction  sets  in.  If, 
therefore,  a  beam  that  has  b^cn  rendered 
conical  by  a  converging  lens  be  rent  through 
the  spat  so  that  the  central  ray  c  f  the  cone 
passes  along  the  axis,  this  ray  or;\y  will  es- 
cape double  refraction.  Each  of  the  others 
will  be  divided  into  an  ordinary  and  extraor- 
dinary ray,  the  one  moving  morf  slowly 
through  the  crystal  than  the  other  ;  the  one, 
therefore,  retarded  with  reference  to  the 
other.  Here,  then,  we  have  the  condition/ 


34 


SIX  LECTURES  ON  LIGHT. 


for  interference,  when  the  waves  are  reduced 
by  the  analyzer  to  a  common  plane.  A 
highly  beautiful  and  important  source  of 
chromatic  phenomena  is  thus  revealed. 
Placing  the  plate  of  spar  between  the  crossed 
prisms,  we  have  upon  the  screen  a  beautiful 
system  of  iris  rings  surrounding  the  end  of 
the  optic  axis,  the  circular  bands  of  color 
being  intersected  by  a  black  cross.  The 
arms  of  th:s  cross  are  parallel  to  the  two 
directions  of  vibration  in  the  polarizer  and 
analyzer.  It  is  easy  to  see  that  those  rays 
whose  planes  of  vibration  within  the  spar 
coincide  with  the  plane  of  vibration  of  cither 
prism,  cannot  get  through  both.  This  com- 
plete interception  produces  the  arms  of  the 
cross.  With  mono-chromatic  light  the  rings 
would  be  simply  bright  and  black — the 
bright  rings  occurring  at  those  thicknesses  of 
the  spar  which  cause  the  rays  to  conspire  ; 
the  black  rings  at  those  thicknesses  which 
cause  them  to  quench  each  other.  Here, 
however,  as  elsewhere,  the  different  lengths 
of  the  light-waves  give  rise  to  iris-colors 
when  white  light  is  employed. 

Besides  the  regular  crystals  which  produce 
double  refraction  in  no  direction,  and  the 
««/</.*•«/ crystals  which  produce  it  in  all  direc- 
tions but  one,  Brewster  discovered  that  in  a 
large  class  of  crystals  there  are  two  directions 
in  which  double  refraction  does  not  take 
place.  These  are  called  biaxal  crystals. 
\\henplatesot  '"hese  crystals,  suitably  cut, 
arc  plac.  d  bet  wee  the  polarizer  and  analyzer, 
ihe  axes  are  sce-i  si  "ounded,  not  by  circles, 
but  by  curv  s  of  anv  '^er  order  and  of  a  per- 
fectly definite  mathem.  'cal  character.  Each 
band  as  proved  experimentally  by  Herschel, 
forms  a  lefnuiscata ;  but  the  experimental 
proof  was  here,  as  in  numberless  other  cases, 
preceded  by  the  deduction  which  showed  that, 
according  to  the  undulatory  theory,  the  bands 
must  possess  this  special  character. 

I  have  taken  this  somewhat  wide  range 
over  polarization  itseli  and  over  the  phenom- 
ena exhibited  by  crystals  in  polarized  light, 
in  order  to  give  you  some  notion  of  the  firm- 
ness and  completeness  of  the  theory  which 
grasps  them  all.  Starting  from  the  single 
assumption  of  transverse  unckilations,  we 
first  of  all  determine  the  wave-lengths,  and 
find  all  the  phenomena  of  color  dependent 
on  this  element.  The  wave-lengths  may  be 
determined  in  many  independent  ways,  and, 
when  the  lengths  so  determined  are  compared 
together,  the  strictest  agreement  is  found  to 
exist  between  them.  We  follow  the  ether 
into  the  most  complicated  cases  of  interac- 
tion between  it  and  ordinary  matter,  "the 
theory  is  equal  to  them  all.  It  makes  not  a 
single  new  hypothesis  ;  but  out  of  its  original 
stock  of  principles  it  educes  the  counterparts 
of  all  that  observation  shows.  It  accounts 
for,  explains,  simplifies  the  most  entangled 
cases  ;  corrects  known  laws  and  facts  ,  pre- 
dicts and  discloses  unknown  ones  ;  becomes  ; 
the  guide  of  its  former  teacher  Observation  ;  ! 


and,  enlightened  by  mechanical  conceptions, 
acquires  an  insight  which  pierces  through 
shape  and  color  to  force  and  cause.  '* 

But,  while  I  have  thus  endeavored  to  illus- 
trate before  you  the  power  of  the  undulatory 
theory  as  a  solver  of  all  the  difficulties  of 
optics,  do  I  there! ore  wish  you  to  close  your 
eyes  to  any  evidence  that  may  arise  against 
it?  By  no  means.  You  may  urge,  and 
justly  urge,  that  a  hundred  years  ago  another 
theory  was  held  by  the  most  eminent  men, 
and  that,  as  the  theory  then  held  had  to 
yield,  the  undulatory  theory  may  have  to 
yield  also.  This  is  perfectly  logical ;  but  let 
us  understand  the  precise  value  of  the  argu- 
ment. In  similar  language  a  person  in  the 
time  of  Newton,  or  even  in  our  time,  might 
reason  thus  :  "  Hipparchus  and  Ptolemy, 
and  numbers  of  great  men  after  them,  be- 
lieved that  the  earth  was  the  centre  of  the 
solar  system.  But  this  deep-set  theoretic 
notion  had  to  give  way,  and  the  theory  of 
gravitation  may,  in  its  turn,  have  to  give 
way  also."  This  is  just  as  logical  as  the  first 
argument.  Wherein  consists  the  strength  of- 
the  theory  of  gravitation  ?  Solely  in  its  com- 
petence to  account  for  all  the  phenomena  of  the 
solar  system.  Wherein  consists  the  strength 
of  the  theory  of  undulation  ?  Solely  in  its 
competence  to  disentangle  and  explain  phe- 
tnomena  a  hundred-fold  more  complex  than 
:hose  of  the  solar  system.  Be  as  skeptical, 
"f  you  like,  regarding  the  undulatory  theory  ; 
but  if  your  skepticism  be  philosophical,  it 
will  wrap  the  theory  of  gravitation  in  the 
same  or  greater  doubt. f 


LECTURE  V. 

Range  of  vision  incommensurate  with  Range  of  Radi- 
ation ;  The  Ultraviolet  Rays:  Fluorescence: 
Rendering  Invisible  Rays  visible :  Vision  not  the 
only  Sense  appealed  to  by  the  Solar  and  Electric 
Beam  :  Heat  of  Beam  :  Combustion  by  Total  Beam 
at  the  Foci  of  Mirrors  and  Lenses:  Combustion 
through  Ice-Lens:  Ignition  of  Diamond:  Search 
for  the  Rays  here  effective  :  Sir  iVllliam  Herschel's 
Discovf ry  of  Dark  Solar  Rays:  Invisible  Rays  the 
Basis  of  tne  Visible  :  Detachment  by  a  Ray-Filter 
of  the  Invisible  Rays  from  the  Visible;  Combustion 
at  Dark  Foci:  Conversion  of  Heat-Rays  into 
Light-Rays  :  Calorescence  :  Part  played  in  Nature 
by  Dark  Rays:  Identity  of  Light  and  Radiant 
Heat:  Invisible  Images:  Reflection,  Refraction, 
Plane  Polarization,  Depolarization,  Circular  Polar- 
ization, Double  Refraction,  and  Magnetization  of 
Radiant  Heat. 

THE  first  question  that  we  have  to  con- 
sider to-night  is  this  ,  Is  the  eye,  as  an  organ 
of  vision,  commensurate  with  the  whole 
range  of  solar  radiation — is  it  capable  of  re- 
ceiving visual  impressions  from  all  the  rays 
emitted  by  the  sun  ?  The  answer  is  nega- 
tive. If  we  allowed  ourselves  to  accept  for  a 


*  Whewell. 

t  The  only  essay  known  to  me  on  the  Undulatory 
Theory,  from  the  pen  of  an  American  writer,  is  an 
excellent  one  by  President  Barnard,  published  in  the 
Smithsonian  Report  for  1862. 


SIX  LECTURES  ON   LIGHT. 


moment  thav  :-oaon  o<"  gradual  growth, 
artiehoi avion,  arid  asconaion,  implied  by  the 
ttrm  e-cohitton,  we  might  lairly  conclude  that 
th»re  arcs'.ortj  of  visual  i  mpressions  a  waiting 
ma  i  far  greater  than  those  ot  which  he  is 
now  in  possession.  For  example,  here  be- 
yond the  extreme  violet  of  the  spectrum  ihere 
is  a  vast  efflux  or  rays  which  are  totally  use- 
less as  regards  our  present  powers  of  \isicn. 
But  these  ultra-violet  waves,  though  incom- 
petent to  awaken  the  optic  nerve,  can  so 
shake  the  molecules  of  certain  compound  sub- 
stances as  to  effect  their  decomposition.  The 
grandest  example  of  the  chemical  action  of 
light,  with  which  my  friend  Dr.  Draper  has 
so  indissolubly  associated  his  name,  is  that  of 
the  decomposition  of  carbonic  acid  in  the 
leaves  of  plants.  All  photography  is  founded 
on  such  actions.  There  are  substances  on 
which  the  ultra-violet  waves  exert  a  special 
decomposing  power  ;  and,  by  permitting  the 
invisible  spectrum  to  fall  upon  surfaces  pre- 
pared with  such  substances,  we  reveal  both 
the  existence  and  the  extent  of  the  ultra- 
violet spectrum. 

This  mode  of  exhibiting  the  action  of  the 
ultra-v.olet  rays  has  be^n  long  known  ;  in- 
deed, Thomas  Young  photographed  the  ultra- 
violet rings  of  Newton.  We  have  now  to 
demonstrate  their  presence  in  another  way. 
As  a  general  rule,  bodies  transmit  light  or 
absorb  it,  but  there  is  a  third  case  in  which  the 
light  falling  upon  the  *x>dy  is  neither  trans- 
mitted nor  absorbed,  but  converted  into  light  of 
another  kind.  Professor  Stokes,  the  occupant 
of  the  Chair  of  Newton  in  the  University  of 
Cambridge,  one  of  those  original  workers 
who,  though  not  widely  known  beyond 
scientific  circles,  really  constitute  the  core  of 
science  has  demonstrated  this  change  of  one 
kind  of  light  into  another,  and  has  pushed 
his  experiments  so  far  as  to  render  the  iuvisi 
'•As  rays  visible. 

A  ion^  list  of  substances  examined  by 
Stokes  when  excited  by  the  invisible  ultra- 
violet waves,  have  been  proved  to  emit  light. 
You  know  the  rate  of  vibration  corresponding 
to  the  extreme  violet  of  the  spectrum  ;  you 
are  aware  that,  to  produce  the  impression  of 
this  color,  the  retina  is  struck  789  millions  of 
millions  of  times  in  a  second.  At  this  point, 
the  retina  ceases  to  be  useful  as  an  orga  i  of 
vision,  for,  though  struck  by  waves  of  more 
rapid  recurr  nee,  they  ?_r«  incompetent  to 
awaken  the  sensation  of  light.  But,  when 
such  non-visual  waves  are  caused  to  impinge  i 
upon  the  molecules  of  certain  substances — •  I 
en  those  of  sulphate  of  quinine,  for  example 
— they  compel  those  molecules,  or  their  con- 
stituent atoms,  to  vibrate  ;  and  the  peculiar- 
ity is,  that  the  vibrations  thus  set  up  are  of 
slower  period  than  those  of  the  exciting 
waves.  By  this  lowering  of  t  c  rate  of  vi- 
oration  through  the  intermediation  of  the  sul- 
phate of  quinine,  the  invisible  rays  are  ren- 
dered visible.  He-e  we  have  our  spectrum, 
and  beyond  the  violet  I  place  this  prepared 


paper.  The  spectrum  is  immediately  elonga- 
ted by  the  generation  of  a  new  light  beyond 
the  extreme  violet.  President  Morton  has 
recently  succeeded  in  discovering  a  substance 
of  great  sensibility  which  he  has  named 
Thallene,  and  he  has  been  good  enough  ro 
favor  me  with  some  paper  saturated  with  a 
solution  of  this  substance.  It  causes  a  very 
striking  enlongation  of  the  spectrum,  the 
!  new  light  generated  being  of  peculiar  bril- 
liancy.  To  this  change  of  the  rays  from  a 
higher  to  a  lower  refrangibility,  Stokes  has 
given  the  name  of  Fluorescence. 

By  means  of  a  deeply-colored  violet  glass, 
we  cut  off  almost  the  whole  of  the  light  of 
our  electric  bean?  ;  but  this  glass  is  peculiarly 
transparent  to  the  violet  and  ultra-violet 
rays.  The  violet  beam  now  crosses  a  large 
jar  filled  with  water.  Into  it  I  pour  a  solu- 
tion of  sulphate  of  quinine :  opaque  clouds, 
to  all  appearance,  instantly  tumble  down- 
wards.  But  these  are  not  clouds  :  there  is 
nothing  precipitated  here  :  the  observed  ac- 
tion is  a  action  of  molecules,  not  of  particles. 
The  medium  before  you  is  not  a  turbid  me- 
dium, for,  when  you  look  through  it  at  a 
luminious  surface,  it  is  perfectly  clear.  If  we 
paint  upon  a  piece  of  paper  a  flower  or  a 
bouquet  with  the  sulphate  of  quinine,  and  ex- 
pose it  to  the  full  beam,  scarcely  anything  is 
seen.  But  on  interposing  the  violet  glass, . 
the  design  instantly  flashes  forth  in  strong 
contrast  with  the  deep  surrounding  violet. 
Here  is  such  a  design  prepared  for  me  by 
President  Morton  with  his  thallene  :  placed 
in  the  violet  light  it  exhioits  a  peculiarly 
vivid  and  beautiful  fluorescence.  From  the 
experiments  of  Dr.  Bence  Jones,  it  would 
seem  that  there  is  some  .substance  in  the  hu- 
man body  resembling  the  sulphate  of  quini.ie, 
which  causes  ail  the  tissues  of  the  body  to  be 
more  or  less  fluorescent.  The  crystalline 
lens  of  the  eye  exhibits  the  effect  in  a  very 
striking  manner.  When  I  plunge  my  ey* 
into  this  violet  beam,  I  am  conscious  of  a 
whitish-blue  shimmer  filling  the  space  before 
me.  This  is  caused  by  fluorescent  light  gen- 
erated in  the  eye  itself  ;  looked  at  from  with- 
out,  the  crystalline  lens  at  the  same  time 
gleams  vividly. 

<*• 

But  the  waves  from  our  incandescent  car- 
bon-points appeal  to  another  sjnse  than  that 
of  vision.  They  not  only  produce  light  as  a 
sensation;  they  also  produce  heat.  The  mag- 
nified image  of  the  carbon  points  is  now  upon 
the  screen,  and  with  a  suitable  instrument  the 
heating  power  of  that  instrument  might  be 
demonstrated.  Here,  however,  the  heat  is 
spread  over  too  large  an  area  to  be  intense. 
By  pushing  out  the  lens  and  causing  a  mova- 
ble screen  to  approach  our  lamp,  the  im  >ge 
becomes  smaller  and  smaller  :  the  rays  be- 
come more  concentrated,  until  finally  they 
are  able  to  pierce  black  paper  with  a  burning 
ring.  Rendering  the  beam  parallel,  and  re- 
ceiving it  upon  a  concave  mirror,  the  rays  are 


SiX  LECTURES  ON  LIGHT. 


brought  to  a  focus  ;  and  paper  placed  at  the  through  a  glass  cell  containing  water.  The 
focus  is  caused  to  smoke  and  burn.  This  beam  is  thus  sifted  of  constituents,  which,  if 
may  be  done  by  our  common  camera  with  its  permitted  lo  fall  upon  the  lens,  would  injure 
lens,  and  by  a  concave  mirror  of  very  mod-  its  surface,  and  blur  the  focus.  And  this 
erate  po\v  r.  leads  me  to  say  an  anticipate! y  word  regard- 

\\  e  v\i,l  row  adopt  stronger  measures  with  ing  transparency.  In  our  first  lecture  we 
the  radiatio. i  from  the  electric  la-rip  In  this  entered  fully  into  the  production  of  colors 
carm  ra  of  blackened  ti  i  is  placed  a  lamp,  in  by  absorption,  and  we  spoke  repeatedly 
all  particulars  similar  to  those  already  em-  of  the  quenching  of  the  rays  of  light  Did 
ployed.  But,  instead  of  gathering  up  the  this  mean  that  the  light  was  altogether 
/ays  from  a  carbon-point  by  a  condensing  annihilated?  By  1,0  means.  It  was  sim- 
!ens  placed  in  front  of  them,  we  gather  them  ply  so  lowered  in  relrangibility  as  to 
up  by  a  co  :cave  mirror,  silvered  in  front,  escape  the  vi-ual  range.  It  was  converted 
and  placed  behind  tLe  carbons.  By  this  into  heat.  Our  red  nbbon  in  the  green  of  the 
mirror  \ve  can  cause  the  rays  t  >  issue  through  spectrum  quenched  the  green,  but  if  suitably 
the  orifice  in  front,  either  parallel  or  conver-  examined  its  tcm  erature  would  have  been 
gent.  They  are  r\ow  prvallel,  and  therefore  found  laised.  Oar. green  ribbon  in  the  red 
to  a  certain  extent  diffu-ed.  We  place  a  of  the  spectrum  quenched  the  red,  but  its 
convex  lens  in  the  path  of  the  beam  ;  the  temperature  at  the  same  lime  was  augmented 
light  is  converged  to  a  focus,  and  at  that  to  a  degree  exactly  tquivalent  to  the  light  ex- 
focus  you  s  e  that  paper  is  not  only  pi.roed  tinguished.  Our  black  ribbon,  when  passed 
and  a  burning  ring  foimed,  but  that  it  is  in-  through  the  spectrum,  was  found  competent 
stantly  set  ablaze.  Many  metals  may  be  to  quench  ail  its  colors  ;  but  at  every  stage  of 
burned  up  in  the  same  way.  In  our  first  lee-  its  progress  an  amount  ot  heat  was  geneiated 
ture  thecombustibility  of  zinc  was  mentioned,  in  the  ribbon  exactly  equivalent  to  the  light 
Macing  a  stiip  <>(  sheet  zinc  at  this  focus,  it  j  lost.  It  is  only  ivhen  absorption  takes  place 
is  instantly  ignitid  and  burns  with  its  charnc-  |  tkat  heat  is  thus  produced ;  and  heat  is  always 


teristic  purple  flame.     (In  the  annexed  fi  .u 
m  in'   represents  the  concave  mi-ror,    L  t 


a  result  of  ab  orption. 

Examine  this  water,  then,  in  front  of  the 
lamp,  after  the  beam  lias  passed  a  little  lime 
through  it :  it  is  sensibly  warm,  and,  if  per- 
mitted to  remain  there  iong  enough,  it  may 
be  made  to  boil.  This  is  due  to  the  absorp- 
tion by  ti.e  water  of  a  portion  of  the  electric 
beam.  But  a  certain  poition  passes  through 
unabsorbed,  and  does  not  at  all  contribute  to 
the  heating  of  the  water  Now,  ice  is  also 
transparent  to  the  latter  portion,  and  there- 
fore is  not  melted  by  it  ;  hence,  by  employ- 
ing this  particular  portion  of  the  beam,  we 
are  able  10  keep  our  lens  intact,  and  to  pro- 
duce by  means  of  it  a  shai  ply-defined  focus. 
Placed  at  that  focus,  black  paper  instantly 
burns,  because  the  black  paper  absorbs  the 
light  which  had  passed  through  the  ice-lens 
without  absorption,  la  a  subsequent  lecture, 
we  shall  endeavor  to  penetrate  further  into 
the  physical  meaning  ot  these  and  other  simi- 
lar actions.  I  may  add  to  these  illustrations 
of  heating  power,  the  ignition  cf  a  diamond 
in  oxygen,  by  the  concentrated  beam  of  ihe 
dectric  lamp.  The  diamond,  surrounded  by 
a  hood  of  platinum  to  lessen  the  chilling  due 
to  convection,  is  exposed  at  the  focus.  It  is 


lens,  at   the  focus  C  of  which  combustion  is 
effected).     Dr.    Scoresby   succeeded    in    ex 
ploding   gunpowder   by  the  sun's  r.ys  con- 
verged by  large  lenses  of  ice  ;  toe  same  effect    rapidly  raised  to  a  white  heat,  and  when  re- 
moved from  ihe  focus  continues  to  glow  like 
a  star. 

this    1  eauiiful   lens    of!      Placed  in  the  path  of  the  beam  issuing  from 
the  fucus  of  the  lens  I    our  lamp  is  a  ceil  with  glass  sides  containing 


may  be  produced  with  a  small  Kns,  and  wjgi 
a  terrestrial  source  of  heat.   In  an  iron  mould 


All  the  light  of  the  beau; 


we    have   fashion*,  d 
transparent  ice.     At 

place  a  bit  of  black  paper,  with  a  little  gun-  a  solution  of  alum. 

cotton  folded  up  within  it.   The  paper  ignites  passes  through  this   solution.     The  beam  i^ 

and   the  cotton  explodes.     Strange,  is  it  not,  received  on  a  powerfully  converging  mirrot 

that    the  beam  should   possess  such  heating  silvered  in  front,  and  is  brought  to  a  focus  by 

power  after  having  passed  through  so  cold  a  the  mirror.      You  can  see  the  conical  beam 
substance?                                                                (of    relUcied  li^ht  tracking  itself   through  tae 

In  this  experiment,  you  observe  that,  before  dust   of   the    room.     I  place  at  the  focus  a 

the  beam  reaches  the  ice-lens,  it  has  passed  scrap  of   white   paper  ;    it  glows  there  with 


SIX    LECTURES  ON    LIGHT. 


dazzling  Diigntness,  but  it  is  not  even  charred. 
On  removing  the  alum-cell,  however,  the 
paper  instantly  inflames.  There  must,  there- 
fo-c,  be  something  in  this  beam  besides  its 
light.  The*  light  is  not  absorbed  by  the 
white  paper,  and  therefore  does  not  burn  the 
paper  ;  but  there  is  something  over  and  above 


These  remarks  apply,  not  only  to  the  vis- 
iblr  emission  examined  by  Dr.  Draper,  but 
to  the  invisible  emission  which  preceded 
the  appearance  of  any  ligl  t.  In  thj  emis- 
s  on  from  the  white-hot  platinum  wire  now 
before  you  the  very  waves  exist  with  which 
we  st  tried,  only  their  intensity  has  been  i  i- 


the  'ight  which  is  absorbed  and  which  pro-    creasid  a  thousand-fold  by  the  augmentation 
vokcs  combustion.     What  is  this  something?  I  of  temperature  necessary  to   the   production 
In   the   year    1800   Sir  William    Herschel  j  of  this  white  light,      i  oth  effects  ar  ;  b  mnd 


passed  a  thermometer  through  the  various 
colors  of  the  solar  spectrum,  an  i  marked  the 
rise  of  temperature  corresponding  to  each 
color.  lie  found  the  heating  effect  to  aug- 
ment from  the  violet  to  the  red  ;  he  di  1  not, 
however,  stop  at  the  red.  but  pushed  his 
thermometer  into  the  dark  space  beyond  it 
Here  he  found  the  temperature  actually  higher 
than  in  any  part  of  the  visible  spectrum.  By 
this  important  observation,  he  proved  that 
the  sun  emitted  dark  heat-rays  which  are  en- 
tirely unfit  for  the  purposes  c  f  vision.  The 
subject  was  subsequently  taken  up  by  See- 
beck,  Melloni,  Miilkr,  and  others,  and  within 
the  1  ist  few  years  it  h  -s  been  found  capable 
of  unexpected  expansions  and  applications. 
A  muhod  has  been  cbvised  whereby  the  solar 
or  electric  beam  can  be  so  filtered  as  to  detach 
from  it  and  preserve  intact  this  invisible 
ultr  -red  emission,  while  the  visible  and  ultra- 
violet emissions  are  wholly  intercepted.  We 
are  thus  enabled  to  operate  at  will  upon  the 
purely  ultra-red  waves. 

In  the  heating  of  so. id  bodies  to  incandes- 
cence this  non-visual  emission  is  the  neces-  |  act  of  chemical  uni  n  is   accompanied  by  an 
sary  basis  of  the  visual.     A  platinum  wire  is  I  enormous  diminution  of  its  diathermancv,  or 
stretched  in  front  of  the  table,  and  through  \  perviousness  to  radiant  heat.   The  researches 

which  established  this  result  also  proved  the.. 

elementary    gases    generally    to   be    highly 


together:  in  an  incandescent  solid,  or  in  a 
molten  solid,  you  cannot  have  the  shorter 
waves  without  this  intensification  of  the 
longer  ones.  A  sun  is  possible  only  on 
these  conditions;  hence  Sir  William  Her- 
schel's  discovery  of  the  invisible  ultra-red 
solar  emission. 

The  invisible  heat,  emitted  both  by  dark 
brdiesand  by  luminous  ones,  flies  through 
space  with  the  velocity  of  light,  and  is  culled 
radiant  heat.  Now,  radiant  heat  may  b; 
made  a  subtle  and  powerful  explorer  of 
molecular  condition,  and  of  late  years  it 
has  given  a  new  significance  to  the  art  of 
chemical  combination.  Take,  for  exam  pi  •, 
the  air  we  breathe.  It  is  a  mixture  of  oxyg  n 
ai.d  nitrogen;  and  with  regard  to  radii. .t 
heat  it  behaves  like  a  vacuum,  being  i.icom 
petent  to  absorb  it  in  any  sensible  deg  er. 
But  permit  the  same  two  gases  to  u  .it; 
chemic  illy;  without  any  augmentation  of  tn  ; 
quantity  of  matter,  without  altering  the  gase 
ous  condition,  without  interfering  i  i  a.iy 
way  with  the  transparency  of  the  g  is,  tlu 


it  an  electric  current  flows.  It  is  warmed  by 
the  current,  and  may  be  felt  to  be  warm  by 
the  hand  ;  it  also  emits  waves  of  heat,  but  no 
1  ght.  Augmenting  the  strength  of  the  cur- 
rent, the  wire  becomes  hotter ;  it  finally 
glows  with  a  sober  red  light  At  this  point 
Dr.  Draper  many  years  ago  began  an  inter- 
esting investigation.  He  employed  a  vol- 
taic current  t:>  heat  his  platinum,  and  he 
studied  by  means  of  a  prism  the  successive 
introduction  of  the  colors  of  the  spectrum. 
His  first  color,  as  here,  was  red  ;  then  came 
orange,  then  yellow,  then  green,  and  lastly 
all  the  shades  of  blue.  Thus  as  the  tempera- 
ture of  the  platinum  was  gradually  aug- 
mented, the  at  ms  were  caused  to  vibrate 
mo:  e  rapidly,  shorter  waves  wer3  thus  pro- 
duced, until  finally  he  obtained  the  waves 


corresponding   to   the   entire  spectrum, 
each    successive   color   was   introduced, 


As 
the 


colors  preceding  it  became  more  vivid.  Now, 
the  vividness,  or  intensity  of  light,  like  that 
of  sound,  depends,  not  upon  the  length  of  the 
wave,  but  on  the  amplitude  of  the  vibration. 
Hence,  as  the  red  grew  more  intense  as  the 
more  refrangible  colors  were  introduced,  we 
ire  forced  to  conclude  that,  side  by  side  with 
the  introduction  of  the  shorter  waves,  we  had 
an  augmentation  of  the  amplitude  of  the 
longer  ones. 


transparent  to  radiant  heat.  This,  again, 
led  to  the  proof  of  the  diathermancy  of  ele- 
mentary liquids,  like  bromine,  an  1  of  solu- 
tions of  the  elements  sulphur,  phosphorus, 
and  iodine.  A  spectrum  is  now  before  you, 
and  you  notice  that  this  transparent  bisul- 
phide of  carbon  has  no  effect  upon  the  colors. 
Dropping  into  the  liquid  a  few  flakes  of 
iodine,  you  see  the  middle  of  the  spectrum 
cut  away.  By  augmenting  the  quantity  of 
iodine,  we  invade  the  entire  spectrum,  and 
finally  cut  it  off  altogether.  Now,  the  iodine 
which  proves  itself  thus  hostile  to  the  light  i-> 
perfectly  transparent  to  the  ultra- red  emis- 
sion with  which  we  have  now  to  deal.  It, 
therefore,  is  to  be  our  ray-filter. 

Placing  the  alum-cell  again  in  front  of  the 
electric  lamp,  we  assure  ourselves,  as  before, 
of  the  utter  inability  of  the  concentrated 
light  to  fire  white  paper.  By  introducing  a 
cell  containing  the  solution  of  iodine,  the 
light  is  entirely  cut  off.  On  remov- 
ing the  alum-eel:,  the  paper  at  the  dark  focus 
is  in.  tantly  set  en  fire.  Black  paper  is  more 
absorbent  than,  white  for  these  ultra-red  rays; 
tuid  the  consequence  is,  that  with  it  the  sud- 
denness and  vigor  of  the  combustion  are  aug- 
mented Zinc  is  burnt  up  at  the  same  place, 


SIX  LECTURES  ON  LIGHT 


while  magnesium  ribbon  bursts  into  vivid 
combustion  A  sheet  of  platinized  platinum 
placed  at  the  focus  is  heated  to  whiteness. 
Looked  at  through  a  prism,  the  white-hot 
platinum  yields  all  the  colors  of  the  spectrum. 
Before  impinging  upon  the  platinum,  the 
waves  were  of  too  slow  recurrence  to  awaken 
vision;  by  the  atoms  of  the  platinum,  these 
long  and  sluggish  waves  are  in  part  broken 
up  into  shorter  ones,  being  thus  brought 
within  the,  visual  range.  At  the  other  end  of 
the  spectrum,  Stokes,  by  the  interposition  of 
suitable  substances,  lowered  the  refrangibil- 
ity  so  as  to  render  the  non- visual  rays  visual, 
and  to  this  change  he  gave  the  name  of 
Fluorescence,  Here,  by  the  intervention  of 
the  platinum,  the  refrangibility  is  raised,  so 
as  to  render  the  non-visual  visual,  and  to 
this  change  we  give  the  name  of  Ca lores 
fence. 

At  the  perfectly  invisible  focus  where  these 
effects  are  produced,  the  air  may  be  as  cold 
as  ice.  Air,  as  already  stated,  does  not  ab- 
sorb the  radiant  heat,  and  is  therefore  not 
warmed  by  it.  Place  at  the  focus  the  most 
sensitive  air-thermometer  :  it  is  not  affected 
by  th.e  heat.  Nothing  could  'more  forcibly 
illustrate  the  isolation,  if  I  may  use  the  term, 
of  the  luminiferous  ether  from  the  air.  The 
wave-motion  of  the  one  is  heaped  up,  without 
sensible  effect,  upon  the  other.  I  may  add 
iliat,  with  suitable  precautions,  the  eye  may 
bj  placed  in  a  focus  competent  to  heat  plati- 
nu into  vivid  redness,  without  experiencing 
any  damage,  or  the  slightest  sensation  either 
of  li^ht  or  heat. 

These  ultra-red  rays  play  a  most  important 
part  in  Nature.  I  remove  the  iodine  filter, 
and  concentrate  the  total  beam.  A  test-tube 
containing  water  is  placed  at  the  focus  :  it 
immediately  begins  to  sputter,  and  in  a  min- 
ute or two  it  boils.  What  boils  it?  Placing 
the  alum  solution  in  front  of  the  lamp,  the 
boiling  instantly  ceases.  Now,  the  alum  is 
pervious  to  all  the  luminous  rays;  hence  it 
cannot  be  these  rays  that  caused  the  boiling. 
I  now  introduce  the  iodine,  and  remove  the 
alum ;  vigorous  ebullition  immediately  re- 
commences. So  that  we  here  fix  upon  the 
invisible  ultra-red  rays  the  heating  of  the  wa- 
ter. We  are  enabled  now  to  understand  the 
momentous  part  played  by  these  rays  in  Na- 
ture. It  is  to  them  that  we  owe  the  warming 
and  the  consequent  evaporation  of  the  tropi- 
cal ocean  ;  it  is  to  them,  therefore,  that  we 
owe  our  rains  and  snows.  They  are 
absorbed  close  to  the  surface  of  the 
ocean,  and  warm  the  superficial  water 
while  the  luminous  rays  plunge  to 
great  depths  without  producing  any 
sensible  effect.  Further,  here  is  a  large  fla^k 
containing  a  freezing  mixture.  T,ue  aqueous 
vapor  of  the  air  has  been  condensed  and 
frozen  on  the  flask,  which  is  now  covered 
with  a  white  fur.  Introducing-the  alum-c  II,  ( 
we  place  the  coating  of  hoar-frost  at  the  in- 
tensely luminous  focus  ;  not  a  spicul  i  r>*  fV 


,  frost  is  melted.  Introducing  the  iodine-cell, 
and  removing  the  alum,  a  broad  space  of  the 
frozen  coating  is  instantly  removed.  Hence 
we  infer  that  the  ice  which  feeds  the  Rhone, 
the  Rhine,  and  othei  Divers  which  have 
glaciers  for  t:~.eir  sources,  is  released  from  its 
imprisonment  upon  the  mountains  by  the 
invisible  ultra- red  rays  of  the  sun. 

The  growth  of  science  is  organic.  The 
end  of  to-day  becomes  to-morrow  the  means 
to  a  remoter  end.  Every  new  discovery  is 
immediately  made  the  basis  of  other^discov- 
eries,  or  of  new  methods  of  investigation. 
About  fifty  years  ago,  CErsted,  of  Copen- 
hagen, discovered  the  deflection  of  a  mag- 
netic needle  by  an  electric  current ;  and 
Thomas  Seebeck,  of  Berlin,  discovered  that 
electric  currents  might  be  derived  from  heat. 
Soon  afterwards  these  discoveries  were  turned 
to  account  by  Nobilt  and  Melloni  in  the  con- 
struction of  an  apparatus  which  has  vastly 
augmented  our  knowledge  of  radiant  heat. 
The  instrument  is  here.  It  is  called  a  thermo- 
electric pile;  and  it  consists  of  thin  bars  of 
bismuth  and  antimony  soldered  together  in 
pairs  at  their  ends,  but  separated  from  each 
other  elswhere.  From  the  ends  of  this  '  'pile" 
wires  pass  to  a  coil  of  covered  wire,  within 
and  above  which  are  suspended  two  magnetic 
needles  joined  to  a  rigid  system,  and  carefully 
defended  from  currents  of  air.  The  heat, 
then,  acting  on  the  pile,  produces  an  electric 
current  ;  t'he  current,  passing  through  the 
coil,  deflects  the  needles,  and  the  magnitude 
of  the  deflection  may  be  made  a  measure  of 
the  heat.  The  upper  needle  moves  over  a 
graduated  dial  far  too  small  to  be  seen.  It 
is  now,  however,  strongly  illuminated.  Above 
it  is  a  lens  which,  if  permitted,  would  form 
an  image  of  the  needle  and  dial  upon  the 
ceiling,  where,  however,  it  could  not  be  con- 
veniently seen.  The  beam  is  therefore  re- 
ceived upon  a  looking-glass,  placed  at  the 
proper  angle,  which  throws  the  image  upon 
the  screen.  In  this  way  the  motions  of  this 
small  needle  may  be  made  visible  to  you  all. 

The  delicacy  of  this  instrument  is  such 
that  in  a  room  like  this  it  is  exceedingly  dif- 
ficult to  work  with  it.  My  assistant  stands 
several  :eet  off.  I  turn  the  pile  towards  him: 
the  heat  from  his  face,  even  at  this  distance, 
produces  a  deflection  of  90°.  I  turn  the  in- 
strument  towards  a  distant  wall,  which  I 
judge  to  be  a  little  below  the  average  temper- 
ature of  the  room.  The  needle  descends  and 
passes  to  the  other  side  of  zero,  declaring  by 
this  negative  deflection  that  the  pile  feels  the 
chill  of  the  wall  Possessed  of  this  instrument, 
of  our  ray-filter,  and  of  our  large  Nicol  prisms, 
we  are  in  a  condition  to  investigate  a  subject 
of  great  philosophical  interest,  and  which 
long  engaged  the  attention  of  some  of  our 
foremost  scientific  workers,  Forbes  being  the 
first  successful  one — the  substantial  identity 
of  light  and  radiant  heat. 

That  they  are  identical  in  all  respects  can- 
not of  course  be  the  case,  for  if  they  were 


SIX  LECTURES  ON  LIGHT. 


39 


they  would  act  in  the  same  manner  upon  all 
instruments,  the  eye  included.  The  identity 
meant  is  such  as  subsists  between  one  color 
and  another,  causing  them  to  behave  alike  as 
regards  reflection,  refraction,  double  refrac- 
tion, and  polarization.  As  regards  reflection, 
we  may  employ  the  looking  glass  used  in  our 
first  lecture.  Marking  any  point  in  the  track 
of  the  reflected  beam,  and  cutting  off  the 
light  by  the  iodine,  on  placing  the  pile  at  the 
marked  point,  the  needle  immediately  starts 
aside.  This  is  true  for  every  position  of  the 
mirror.  So  that  both  for  light  and  heat  the 
same  law  of  reflection  holds  good;  for  both 
of  them  also  the  angular  velocity  of  the  re- 
flected beam  is  twice  that  of  the  reflecting 
mirror.  Receiving  the  beam  on  a  concave  I 
mirror,  it  is  gathered  up  into  a  cone  of'  re-  I 
fleeted  >ight;  marking  the  apex  of  the  cone,  | 
and  cutting  off  the  light,  a  moment's  expo- 
sure of  the  pile  at  the  marked  point  produces 
a  violent  deflection  of  the  needle.  (See  Fig. 
23,  where  m  m  is  the  mirror,  P  the  pile,  and 
T  the  opaque  solution.) 

This  beam  of  light  now  enters  a  right- 
angled  prism  and  is  reflected  at  the  hypothe- 
nuse,  in  a  direction  perpendicular  to  its  for- 
mer one.  The  reflection  here  is  total.  Cut- 
;ing  off  the  light,  we  prove  the  reflection  of 
the  heat  to  be  total  also.  The  formation  of 


}J,  <',  Fig.  24,  and  placing  oui  pile  D  be- 
hind  the  analyzer,  neither  heat  nor  light 
reaches  it;  the  needle  remains  undeflect- 
ed.  Introducing  the  iodine,  the  slight- 
est turning  of  either  prism  causes  the 
heat  to  pass,  and  to  announce  it- 
self by  the  deflection  of  the  needle.  Like 
light,  therefore,  heat  is  polarized.  Crossing 
the  Nicols  again,  the  heat  is  intercepted  and 


invisible  images  by  lenses  and  mirrors  may 
also  be  demonstrated.  Concentrating  the 
beam,  and  cutting  off  the  light,  at  the  dark 
focus  the  carbon-points  burn  their  images 
through  a  sheet  of  black  paper.  Placing  a 
sheet  of  platinized  platinum  at  the  focus, 
when  the  concentration  is  strong  an  incan- 
descent image  of  the  points  is  immediately 
stamped  upon  the  platinum. 

And  now  for  polarization  and  its  attendant 
phenomena.     Crossing  our  two  Nicol  prisms, 


the  needle  returns  to  zero.  Plunging  into 
the  dark  space  between  the  prisms  our  plate 
of  mica,  the  needle  instantly  starts  off,  show- 
ing that  the  mica  acts  upon  the  heat  as  it  did 
upon  the  light  :  we  have  in  both  cases  the 
same  resolution  and  recompounding  of  vi- 
brations. Removing  the  mica,  the  needle 
falls  to  zero  ;  but,  on  introducing  a  plate  of 
quartz  between  the  prisms,  the  consequent 
deflection  declares  the  circular  polarization  of 
the  heat.  For  double  refraction  it  is  neces- 
sary that  our  images  should  not  be  too  large 
and  diluted  :  here  are  the  two  disks  pro- 
duced by  the  splitting  of  the  beam  in  Jce- 
land  spar.  Marking  the  positions  of  the  disks 
and  cutting  off  the  light,  the  pile  finds  in  its 
places  two  heat-images.  The  needle  now 
stands  near  90°,  and,  on  turning  the  spar, 
the  deflection  remains  constant.  Transfer- 
ring the  pile  to  the  other  image,  the  deflec, 
tion  of  90°  is  maintained  ;  but  on  turning 
the  spar  the  needle  now  falls  to  zero.  The 
reason  is  manifest!  Permitting  the  light  to 
pass,  we  find  the  luminous  disk  at  some  dis- 
tance from  the  pile.  We  are  dealing,  in 
fact,  with  the  extraordinary  beam  which 
rotates  round  the  ordinary  So  that  fcr  heat 
as  well  as  for  light  we  have  double  refraction, 
and  also  an  ordinary  and  extraordinary  ray. 
(In  the  adjacent  figure,  which  shows  the  ex- 
perimental arrangement,  N  is  the  nozzle  of 


40 


SIX  LECTURES  ON  LIGHT. 


the  electric  lamp.  1  a  converging  lens,  B  the 
^refracting  spar,    and  F  the  thermo-electric 

fi'i  time  permitted  we  might  finish  the  series 
<5r  demonstrations  •  by  magnetizing  a  ray  of 
heat  as  we  magnetized  a  ray  of  light. 

We  have  finally  to  determine  the  position 
and  magnitude  of  the  invisible  ••adiatiou 
which  produces  these  results.  For  this  pur- 
pose we  employ  a  particular  form  of  the 
thermo-electric  pile.  Its  face  is  a  rectangle, 
which  by  movable  side-pieces  can  be  ren- 
dered as  narrow  as  desirable.  Throwing  a 
concentrated  spectrum  upon  a  screen, 
by  means  of  an  endless  screw,  we  move  this 
rectangular  pile  through  the  entire  spectrum. 
Its  surface  is  blackened  so  that  it  absorbs  all 
the  light  incident  upon  it,  converting  it  into 


a  curve  which  exhibits  the  distribution  of 
heat  in  our  spectrum.  It  is  represented  in 
the  adjacent  figure.  Beginning  at  the  blue, 
the  curve  rises,  at  first  very  gradually;  then, 
as  it  approaches  the  red  more  rapidly,  the 
line  CD  representing  the  strength  of  the  ex- 
treme red  radiation.  Beyond  the  red  it  shoots 

!  upwards  in  a  steep  and  massive  peak  to  B, 
whence  it  falls,  rapidly  for  a  time,  and  after- 

!  wards  gradually  fading  from  the  perception 
of  the  pile.  This  figure  is  the  result  of 
more  than  twelve  careful  series  of  measure- 
ments, for  each  of  which  the  curve  was  con- 
structed. On  superposing  all  these  curves, 
a  satisfactory  agreement  was  found  to  exist 
between  them.  So  that  it  may  safely  be 
concluded  that  the  areas  of  the  dark  and 
white  spaces  respectively  represent  the  rela- 


FIG.  2$. 


heat*  and  thus  enabling  it  to  declare  its  power 
by  the  deflection  of  the  magnetic  needle. 

When  this  instrument  is  brought  to  the 
violet  end  of  the  spectrum,  the  heat  is  found 
to  be  almost  insensible.  As  the  pile  grad- 
rally  moves  from  the  violet  towards  the  red; 
it  encounters  a  gradually  augmenting  heat. 
The  red  itself  possesses  the  highest  heating 
power  of  all  the  colors  of  the  spectrum. 
Pushing  the  pile  into  the  dark  space  beyond  the 
red,  the  heat  rises  suddenly  in  intensity, and, at 
some  distance  beyond  the  red,  attains  a  max- 
imum. From  this  point  the  heat  falls  some- 
what more  rapidly  than  it  rose,  and  afterwards 
gradually  fades  away.  Drawing  an  hori- 
zontal line  to  represent  the  length  of  the 
spectrum,  and'  erecting  along  it,  ft  viYi-nu 
points,  perpendiculars  proportional  H  length 
to  the  heat  existing  at  those  points,  we  obtain 


I  live  energies   of    the    visible    and  invisible 
I  radiation      The  one  is  7.7  times  the  other. 

But  in  verification,  as  already  stated,  con- 
sists the  strength  of  science  Determining 
in  the  first  place  the  total  emission  from  the 
electric  lamp;  then  by  means  of  the  iodine 
filter  determining  the  ultra  red  emission  ;  the 
difference  between  both  gives  the  luminous 
emission.  In  this  way,  it  was  found  that  the 
energy  of  the  invisible  emission  is  eight  times 
that  of  the  visible.  No  two  methods  could 
be  more  opposed  to  each  other,  and  hardly 
j  any  two  results  could  better  harmonize.  I 
think,  therefore,  you  may  rely  upon  the  ac- 
curacy of  the  distribution  of  heat  here  as- 
sig  ied  to  the  prismatic  spectrum  of  the  elec- 
tric light.  There  is  nothing  vague  in  the 
mode  of"  investigation,  nor  doubtful  in  its 
conclusions. 


SIX  LECTURES  ON  LIGHT. 


4« 


LECTURE    VI. 

Or\&Mj^*c   of   Spectrum  Analysis:    Solar  Chemistry: 
Summary  and  Conclusions. 

We  have  employed,  as  our  source  of  light 
in  these  lectures,  the  ends  of  two  rods  of  coke 
rendered  incandescent  by  clectricitv.  Coke 
is  particularly  suitable  for  this  purpose,  be- 
cause it  can  bear  intense  heat  without  fus  on 
or  vaporization.  It  is  also  black,  which 
helps  the  light;  for,  other  circumstances  br- 
ing equal,  as  shown  experimentally  by  Bal- 


four  Stewart,  the  blacker  the  body  the 
brighter  will  be  its  light  when  incandescent. 
Still,  refractory  as  carbon  is,  if  we  closely  ex- 
amine our  voltaic  arc,  or  stream  of  light  be- 
tween the  carbon-points,  we  should  find  ihere 
incandescent  carbon-vapor.  We  might  also 
detach  the  light  of  this  vapor  from  the  more 
dazz'ing  light  of  the  solid  points,  and  obtain 
its  ^ptctrum  This  would  be  not  only  less 
brilliant,  but  of  a  totally  different  character 
from  the  spectra  that  we  have  ;  Iready  seen* 
Instead  of  being  an  unbroken  succession  of 
colors  from  red  to  violet,  the  carbon -vapor 


'  SlA  LECTURES  ON  UC5IJT, 


\vould  yield  a  few  bands  of  color  with  spaces 
of  darkness  between  them. 

What  is  true  of  the  carbon  is  true  in  a  still 
rr.ore  striking  degree  of  the  metals,  the  most 
refractory  of  which  can  be  fused,  boiled,  md 
reduced  to  vapor  by  the  electric  current. 
From  the  incandescent  vapor  the  light,  as  a 
general  rule,  flashes  in  groups  of  rays  of 
definite  degrees  of  refrangibility,  spaces  ex- 
isting between  group  and  group,  which  are 
unfilled  by  rays  of  any  kind.  But  the  con- 
templation of  the  facts  will  render  this  sub- 
ject rr.ore  intelligible  than  words  can  make  it. 
Within  the  camera  is  now  placed  a  cylinder 
of  carbon  hollowed  out  at  the  trp  to  receive 
a  bit  of  metal;  in  the  hollow  is  placed  a  frag- 
ment of  the  mital  thallium,  and  now  you  see 
the  arc  of  incandescent  thallium-vapor  upon 
the  screen.  It  is  of  a  beautiful  green  color. 
What  is  the  meaning  of  that  green?  We 
answer  the  question  by  subjecting  the  light 
to  prismatic  analysis.  Here  you  have  its 
sj  cctrum,  consisting  of  a  single  refracted 
band.  Light  of  one  degree  of  refrangibility, 
and  that  corresponding  to  green,  is  emitted 
by  the  thallium-vapor. 

We  will  now  remove  the  thallium  and  put 
a  bit  of  silver  in  its  place.  The  arc  of  silver 
is  not  to  be  distinguished  from  that  of  thal- 
lium; it  is  not  only  green,  like  the  thallium- 
vapor,  but  the  same  shade  cf  green.  Arc 
they,  then,  alike?  Prismatic  analysis  en 
ables  us  to  answer  the  question.  It  is  per- 
fectly impossible  to  confound  the  spectrum 
<  f  incandescent  silver  vapor  with  that  of 
thallium.  Here  are  two  green  bands  instead 
of  one.  Adding  to  the  silver  in  our  camera 
a  1  it  of  thallium,  we  obtain  the  light  of  both 
metals,  and  you  see  that  the  green  of  the  thal- 
lium lies  midway  between  the  two  greens  of 
the  silver.  Hence  this  similarity  of  color. 

But  you  observe  another  interesting  fact. 
The  thallium  band  is  now  far  brighter  than 
the  silver  bands  ;  indeed,  the  latter  have  won- 
dei  fully  degenerated  since  the  bit  of  thallium 
was  put  in.  They  are  not  at  all  so  bright  as 
they  were  at  first,  and  for  a  reason  worth 
knowing.  It  is  the  resistance  offered  to  the 
passage  of  the  electric  current  from  carbon 
to  carbon  that  calls  forth  the  power  of  the 
current  to  produce  heat.  If  the  resistance 
were  materially  lessened,  the  heat  would  be 
materially  lessened ;  and,  if  all  resistance 
were  abolished,  there  would  be  no  heat  at 
all.  Now,  thallium  is  a  much  more  fusible 
and  vaporizable  metal  than  silver  ;  and  its 
vapor  facilitates  the  passage  of  the  current  to 
such  a  degree  as  to  render  it  almost  incom- 
petent to  vaporize  the  silver.  But  the  thal- 
lium is  gradually  consumed  ;  its  vapor  di- 
minishes, the  resistance  rises,  until  finally 
you  see  the  two  silver  bands  as  brilliant  as 
they  were  at  first.  The  three  bands  of  the 
two  metals  are  now  of  the  same  sensible 
brightness. 

We  have  in  these  bands  a  perfectly  unal- 
terable characteristic  of  these  two  metals. 


You  never  get  other  bands  than  these  two 
green  ones  from  the  silver,  never  other  than 
the  single  green  band  from  the  thallium, 
never  oiher  than  the  three  green  bands  from 
the  mixture  of  both  metals.  Every  known 
metal  has  its  bands,  and  in  no  known  case 
are  the  bands  of  two  different  metals  alike. 
Hence  these  spectra  may  be  made  a  test  for 
the  presence  or  absence  of  any  particular 
metal.  If  we  pass  from  the  metals  to  their 
alloys,  we  find  no  confusion.  Copper  gives 
us  green  bands,  zinc  gives  us  blue  and  red 
bands  ;  brass,  an  alloy  of  copper  and  zinc, 
gives  us  the  bands  of  both  metals,  perfectly 
unaltered  in  position  or  character. 

But  we  are  not  confined  to  the  metals  ;  the 
salts  of  these  metals  yield  the  bands  of  the 
metals.  Chemical  union  is  ruptured  by  a 
sufficiently  high  heat,  the  vapor  of  the  metal 
is  set  free  and  yields  its  characteristic  bands. 
The  chlorides  of  the  metals  are  particularly 
suitable  for  experiments  of  this  character. 
Common  salt,  for  example,  is  a  compound  of 
chlor  ne  and  sodium  ;  in  the  electric  lamp,  it 
yields  the  spectrum  of  the  metal  sodium 
The  chlorides  of  lithium  and  of  strontium 
yield  in  like  manner  the  bands  of  these 
metals.  When,  therefore,  Bunsen  and  Kirch- 
hoff,  the  celebrated  founders  of  spectrum 
analysis,  after  having  established  by  an  ex- 
haustive examination  the  spectra  of  all  known 
substances,  discovered  a  spectrum  containing 
bands  different  from  any  known  bands,  they 
immediately  inferred  the  existence  of  a  new 
metal.  They  were  operating  at  the  time 
upon  a  residue  obtained  by  evaporating  one 
of  the  mineral  waters  of  Germany.  In  thaf 
water  they  knew  the  new  metal  was  con- 
cealed, but  vast  quantities  of  it  had  to  b& 
evaporated  before  a  residue  could  be  obtained 
sufficiently  large  to  enable  ordinary  chemistry 
to  grapple  with  the  metal.  But  they  hunted 
it  down,  and  it  now  stands  among  chemical 
substances  as  the  metal  Rubidium.  They 
subsequently  discovered  a  second  metal, 
whic  i  they  called  Casium.  Thus,  having 
first  placed  spectrum  analysis  on  a  safe  foun- 
dation, they  demonstrated  its  capacity  as  an 
agent  of  discovery.  Soon  af.erwards  Mr. 
Crookes,  pursuing  this  same  method,  ob- 
tained the  salts  of  the  thallium  which  yielded 
that  bright  monoc  romatic  green  band.  The 
metal  itself  was  first  isolated  by  a  French 
chemist. 

All  this  relates  to  chemical  discovery  upon 
earth,  where  the  materials  are  in  our  own 
hands.  But  Kirchhoff  showed  how  spectrum 
analysis  might  be  applied  to  the  investigation 
of  the  sun  and  stars,  and  on  bis  way  to  this 
result  he  solved  a  problem  which  had  been 
long  an  enigma  to  natural  philosophers.  A 
spectrum  is  pure  in  which  the  colors  do  not 
overlap  each  other.  WTe  purify  the  spectrum 
by  making  our  slit  narrow  and  by  augment- 
ing the  number  of  our  prisms.  When  a  pure 
spectrum  of  the  sun  has  been  obtained  in 
this  way  it  is  found  to  be  furrowed  by  in- 


SIX  LECTURES  ON  LIGHT. 


dark  lines.  Four  of  them  were 
first  seen  by  Dr.  Wollaston,  but  they  were 
arterwards  multiplied  anJ  measured  by  Fraun- 
|"-fer  with  such  masterly  skill  that  they  are 
now  universally  known  as  Fraunhofer's  lines. 
To  give  an  explanation  of 
these  lines  was,  as  I  have  said, 
a  problem  which  long  chal- 
lenged the  attention  of  philos- 
ophers. (The  principal  lines 
are  lettered  according  to  Fraun- 
hofer  in  the  annexed  sketch  of 
the  solar  spectrum  A,  it  may 
be  stated  stands  near  the  ex- 
treme red,  and  J  near  the 
extreme  violet.) 

Now,  Kirchhoff  had  made 
thoroughly  clear  to  his  mind 
the  princ  pies  which  linked  to- 
gether the  emission  of  light 
and  the  absorption  of  light  ; 
he  had  proved  their  insepara- 
bility for  each  particular  kind 
of  light  and  heat.  He  had 
proved,  for  every  specific  ray 
of  the  spectrum,  the  doctrine 
that  the  body  emitting  any  ray 
absorbed  with  special  energy  a 
ray  of  the  same  refrangibility. 
Consider,  then,  the  effect  of 
knowledge,  such  as  you  now 
possess,  upon  a  mind  prepared 
like  that  of  Kirchhoff.  We 
have  seen  the  incandescent  va- 
pors of  metals  emitting  defi- 
nite groups  of  rays  ;  accord- 
ing to  Kirchhoff 's  principle, 
those  vapors,  if  crossed  by  solar 
light,  ought  to  absorb  !ays  of 
the  same  refrangibility  as  those 
which  they  emit.  He  proved 
this  to  be  the  case ;  he  was 
able,  by  the  interposition  of  a 
vapor,  to  cut  out  of  the  solar 
spectrum  the  band  correspond- 
ing in  color  to  that  vapor. 
Now.  the  sun  possesses  a  pho- 
tosphere, or  vaporous  enve- 
lope— doubtless  mixed  with  vi- 
olentlv  agitated  clouds — and 
Kirchhoff  saw  that  the  power- 
ful rays  coming  from  the  solid,  or  the 
molten  nucleus  of  the  sun,  must  be  inter- 
cepted by  this  vapor.  One  dark  band  of 
Fraunhof  r,  for  example,  occurs  in  the 
yellow  of  the  spectrum.  Sodium  vapor  is 
.demonstrably  competent  to  produce  that  dark 
band;  hen- e  Kirchhoff  inferred  the  exist- 
ence of  sodium-vapor  in  the  atmosphere  of 
the  sun.  In  the  cas,e  of  metals,  which  emit 
a  large  number  of  bands,  the  absolute  coi 
c  dence  of  every  bright  band  of  the  nutal 
with  a  dark  Fraunhofer  line,  raises  to  tho 
highest  degree  of  certainty  the  inference  that 
the  metal  is  present  in  the  atmosphere  of  the 
sun.  In  this  way  solar  chemistry  was  found- 
*d  on  spectrum  analysis. 


FIG.  27. 


I      But  let  me  not   skim   so    lightly    over  this 
great     subject.      T    have   spoken    of    emis- 
1  sion    and    absorption,  and   of   the    link  that 
I  binds  them.     L<.t  me  endeavor  to  make  plain 
to  you,  through  the  analogy   of   sound,  their 
|  physical  meaning.    I  draw  a  fiddle-bow  aciois 
this  tuning-fork,  and  it    immediately  fills  the 
room  with  a  musical  sound;  this   may  be  re- 
garded as  the  radiation  or   emission  of  soun  1 
from  the  fork.   A  few  days  ago,  on  sounui.ig 
this  fork,  I  noticed  that,  when    its  vibratio  is 
were  quenched,  the  sound  seemed  to  be  con- 
!  tinned,  though  more  feebly.     The  sound  ;  p- 
!  peared  to  come  from    under   a   distant  tabl  , 
|  where  stood  a  number  of  tuning-forks  of  dif- 
j  ferent  sizes  and  rates   of  vibration.     One  of 
these,  and  one  only,  had  been  started  by  the 
fork,  and  it  was  one  whose   rate  of  vibration 
was  the  same   as   that    of  the  fork     which 
started  it.     This  is   an  instance   of  the  ab- 
sorption of   sound  of  one  fork    by   another. 
j  P.acing  two  forks  near  each   other,  sweeping 
j  the  bow  over  one  of  them,  and  then  quench- 
|  ing  the  agitateJ  fork,  the  other  c  -ntinues  to 
j  sound.     Placing   a  cent-piece  on  each  prong 
of  one  of   the  forks,  we   destroy   its    perfect 
synchronism  \\  ith  the  oth,  r,  and  then  no  co.n- 
munication  of  sound  from  the  one  to  the  other 
I  is  possible. 

i  will  now  do  with  //V^what  has  been  here 
done  with  sound.  Placing  a  tin  spoon  con 
taining  sodium  in  a  Bunsen's  flame,  we  ol  - 
tain  this  intensely  yellow  light,  which  corr  . 
sponds  in  refrangibility  with  the  yellow  band 
of  the  spectrum.  Like  our  tuning-fork,  it 
emits  waves  at  a  special  period.  I  will  send 
the  white  light  from  our  lamp  through  that 
flame,  and  prove  before  you  that  the  yellow 
flame  intercepts  the  yellow  of  the  spectrum 
S  S,  Fig.  28;  in  other  words,  absorbs  \v;ves 
of  the  same  period  as  its  own,  t'hus  produc- 
ing, to  all  intents  and  purposes,  a  dark 
j  Fraunhofer's  band  in  the  place  of  the  yellow. 
j  (A  Bunsen's  flame  contained  within  the  chim- 
ney Cis  placed  in  front  of  the  lamp  L.  Th  i 
tin  spoon  with  its  pellet  of  sodium  is  plunged 
into  the  flame.  Vivid  combustion  soon  sets 
in,  and,  whe  n  it  does,  the  yellow  of  the  spec- 
trum,  at  D,  is  furrcwed  by  a  dark  ban  '. 
Withdrawing  and  introducing  the  sodium- 
flame  in  rapid  succession,  the  sudden  disap 
pearance  a  id  reappearance  of  the  strip  of 
dai  kness  aro  observed). 

Mentally,  as  well  as  physically,  every  agt 
j  of  the  world  is  the  outgrowth  and  offspn  g 
of  all  preceding  ages.  Science  proves  itself 
to  be  a  genuine  product  of  Nature  by  grow 
ing  according  to  this  l?w.  We  have  no  so- 
lution of  continuity  here.  Every  great  dis- 
covery has  been  duly  prepared  for  in  two 
ways:  first,  by  other  discoveries  which  form 
its  prelude;  and,  secondly,  through  the 
sharpening,  by  exercise,  of  the  intellectual 
instrument  itself.  Thus  Ptolemy  grew  cue 
of  Hipparchus,  Copernicus  out  of  both,  Kep- 
ler out  of  all  three,  and  Newton  out  of  all 
the  four.  Newton  did  not  rise  suddenly  from 


SIX  LECTURES  ON  LIGHT. 


the  sea-level  of  the  intellect  to  his  amazing 
elevation.  At  the  time  that  he  appeared,  the 
table-land  of  knowledge  was  already  high. 
He  juts,  it  is  true,  above  the  table-Ian-',  as  a 
massive  peak;  still  he  is  supported  by  it,  and 
a  great  part  of  his  absolute  height  was  the 
height  of  humanity  in  his  time.  It  is  thus 


»'tn  the  dib-covx  ~ies  of  Kirchhoff.  Much  had 
beta  previous!/  accomplished;  this  he  mas- 
tered, and  the*:  by  the  force  of  individual 
genius  went  beyond  it.  He  replaced  uncer- 
tainty by  certaiuty,  vagueness  by  definite- 
ness,  confusion  Ly  crder;  and  I  do  not  think 
that  Newton  has  a  sn>  er  claim  to  the  discov- 
eries that  have  made  h's  name  immortal  than 
Kirchhoff  has  to  the  credit  of  gathering  up  the 
fragmentary  knowledge  of  his  time,  of  vastly 
extending  it,  and  of  infusing  into  it  the  life 
of  great  principles.  Splendid  results  have 
since  been  obtained  with  which  the  names  of 
Janssen,  Huggins,  Loci:.)  ?r,  Respighi, 
Young,  and  others,  are  honorably  associated, 
out,  splendid  as  they  are,  they  are  but  the 
sequel  and  application  of  the  principles  es- 
tablished in  his  Heidelberg  laboratory  by  the 
celebrated  German  investigator. 


SUMMARY  AND  CONCLUSION 

My  desire  in  these  lectures  has  bf.n  to 
show  you,  with  as  little  breach  of  continuity 
as  possible,  the  past  growth  and  p/esent  as 
pect  of  a  department  of  science,  n  which 
have  labored  some  of  thj  gre:  test  i  itellccts 
the  world  has  ever  seen.  My  f  "iend  Profes- 
sor Henry,  in  introducing  me  atWaslvn^ton, 
spoke  of  me  as  an  apostle;  but  t.aeonl .  apos- 
tolate  that  I  intended  to  fulfil  was  to  place, 


in  plain  words,  my  subject  before  you,  and  to 
permit  its  own  intrinsic  attr  ctions  to  act  up- 
on your  minds.  In  the  way  of  experiment,  I 
have  tried  to  give  you  the  best  which,  under 
the  circumstances,  could  be  provided;  but  I 
have  sought  to  confer  on  each  experiment  a 
distinct  intellectual  val.ie,  for  experiments 
ought  to  be  the  representatives  and  exposi- 
tors of  thought — a  language  addressed  to  the 
eye  as  spoken  words  are  to  the  ear.  16  as- 
sociation with  its  context,  nothing  is  more 
impressive  or  instructive  than  a  fit  experi- 
ment; but,  apart  from  its  context,  it  rather 
suits  the  conjuror's  purpose  of  surprise  than 
that  purpose  of  education  which  ought  to  be 
the  ruling  motive  of  the  scientific  man. 

And  now  a  brief  summary  of  our  work 
will  not  be  out  of  place.  Our  present  mas- 
tery over  the  laws  and  phenomena  of  light 
has  its  origin  in  the  desire  of  man  to  know. 
We  have  seen  the  ancients  busy  with  this 
problem,  but,  like  a  child  who  uses  his  arms 
aimlessly  for  want  of  the  necessarv  muscular 
exercise,  so  these  early  men  speculated  vague- 
ly and  confusedly  regarding  light,  not  having 
as  yet  the  discipline  needed  to  give  clearness 
to  their  insight,  and  firmness  to  their  grasp 
of  principles.  They  assured  themselves  of 
th ;  rectilineal  propogation  of  light,  and  that 
the  angle  of  incidence  was  equal  to  the  angle 
of  reflection.  For  more  than  a  thousand 
years — I  might  ray,  indeed,  for  more  than 
fifteen  hundred  years  subsequently  —  the 
scientific  intellect  appears  as  if  smitten  with 
'  paralysis,  the  fact  being  that,  during  this 
time,  the  mental  fo'ce,  which  might  have  run 
in  the  direction  of  science,  was  diverted  in- 
.o  other  directions. 

The   course    of   investigation    as    regards 
j  light   was   resumed   in    1100  by  an  Arabian 
!  philosopher  named   A.'hazan.     Then  it   was 
I  taken  up  in  succession  by  Roger  Bacon,  Vi- 
I  tellio,  and  Kepler.     These  men,  though  fail- 
ing to  detect  the  principle  which  ruied  the 
facts,  kept  the  fire  of  investigation  constantly 
j  burning.     Then  came   the  fundamental  dis- 
covery of  Snell,  that  corner-stone  of  optics, 
I  as  I  have  already  called  it,  and  immediately 
afterward   we  have  the  application  by  Des 
cartes  of  Snell's  discovery  to  the  explanation 
of    the   rainbow.       Then     came     Newton's 
crowning   experiments   on  the  analysis    and 
I  synthesis  of   white   light,   by   vhich   it   was 
'  proved   to  be  compounded  of  various  kinds 
of  light  of  different  degrees  of  refrangibiiity. 
In  1676  an  impulse  was  given  to  optics  by 
astronomy.     In  that  year   Olaf    Roemer,    a 
learned  Dane,  was  engaged  at  the  Observa- 
tory of  Paris  in  observing  the  eclipses  of  Ju- 
piter's moons.      He  converted  them  into  so 
many    signal-lamps,    quenched    when    they 
plunged  into  the  shadow  of  the  planet,   and 
relighted  when  they  emerged  from  the  shadow. 
They    enabled     him     to     prove  that     light 
requires  time  to  pass  through   space,  and  to 
j  assign  to  it  the  astounding  velocity  of  190,- 
I  ooo  miles  a  second.     Then  came  the  English 


SIX  LECTURES  OX  LIGHT. 


45 


astronomer,  Bradley,  who  noticed  that  the 
fixed  stars  did  not  really  appear  to  be  fixed, 
but  dvscribe  in  the  heavens  evtry  year  a  little 
orbit  resembling  the  earth's  orbit.  The  re- 
su;t  perplexe?!  him.  but  Bradley  had  a  mind 
open  to  suggestion,  and  capable  of  seeing,  in 
the  smallest  fact,  a  picture  of  the  largest.  He 
was  one  day  upon  ihe  Thames  in  a  boat,  and 
noticed  that,  as  io:i  r  as  his  course  remained 
unchanged,  the  vane  upo  i  his  mast-head 
showed  the  wind  to  be  blowing  constantly  in 
the  same  direction,  but  that  the  wind  ap- 
peared to  vary  with  ev.-ry  change  in  the  di- 
rection of  his  boat.  "  Here,"  as  Whewell 
s  y  ,  "  was  the  image  of  his  case  The  boat 
was  the  earth,  moving  in  its  orbit,  and  the 
vind  was  the  light  of  a  star." 

We  rnay  ask  in  passi.ig,  what,  without  the 
.'acuity  which  formed  the  '"image,"  would 
J5radley's  wind  and  vane  have  been  to  him  ? 
A  wind  and  vane,  and  nothing  more.  You 
will  immediately  understand  the  meaning  of 
BraUey's  discovery.  Imagine  yourself  in  a 
noiionless  railway-train  with  a  shower  of  rain 
descending  vertically  downward.  The  mo- 
ment the  train  begins  to  move,  the  rain-drops 
begin  to  s^ant,  and  the  quicker  the  train  the 
greater  is  the  obliquity.  In  a  precisely  sim- 
ilar manner  the  rays  from  a  star  vertically 
overhead  are  caused  to  slant  by  the  motion  of 
the  earth  through  space.  Knowing  the 
speed  of  the  train,  and  the  obliquity  of  the 
falling  rain,  the  velocity  of  the  drops 
may  be  calculated  ;  and  knowing  the  speed 
of  the  earth  in  ner  orbit,  and  the  obliquity  of 
the  rays  due  to  this  cause,  we  can  calculate 
just  as  easily  the  velocity  of  light.  Bradley 
did  this,  and  the  "aberration  of  light,"  as  his 
discovery  is  called,  enabled  him  to  assign  to 
it  a  velocity  almost  identical  with  that  de- 
duced by  Roe  ner  from  a  totally  different 
method  of  observation.  Subsequently  Fizeau, 
employing  not  planetary  or  stellar  distances, 
but  simply  the  breadth  of  the  city  of  Paris, 
determined  the  veloci  y  of  light  :  while  after 
him  Foucault — a  man  of  the  rarest  mechanical 
genius — solved  the  problem  witn  mt  quitting 
his  private  room. 

Up  to  his  demonstration  of  the  composition 
of  white  light,  Newton  had  been  everywhere 
triumphant — triumphant  in  the  heavens,  tri- 
umphant on  the  earth,  and  his  subsequent 
experimental  work  is  for  the  most  part  of 
immortal  value.  But  infallibility  is  not  the 
gift  of  man,  and,  soon  after  his  discovery  of 
the  nature  of  white  light,  Newton  proved 
himself  human.  He  supposed  that  refraction 
and' dispersion  went  hand  in  hand,  .ana"  that 
you  could  not  abolish  the  one  without  at  the 
same  time  abolishing  the  other.  Here  Dol- 
land  corrected  him  But  Newtoa  committed 
a  graver  error  than  this.  Science,  as  1 
sought  to  make  clear  to  yo.i  in  our  second 
lecture,  is  on!y  i  i  part  a  thing  of  the  senses. 
The  roots  of  piienonuna  are  embedded  in  a 
region  beyond  the  reach  of  the  sen  es,  and 
less  than  the  rout  of  the  matter  uii!  never 


satisfy  the  scientific  mind.  We  find,  accord- 
ingly, in  tiiis  career  of  optics,  the  greater 
minds  constantly  yearning  to  pass  from  the 
phenomena  to  their  causes — to  explore  them 
to  their  hidden  roots.  They  thus  entered 
the  region  of  theory,  and  here  Newton, 
though  drawn  from  time  to  time  towards  the 
truth,  was  drawn  sii.l  more  strongly  towards 
the  error,  anJ  made  it  his  substantial  choice. 
His  experiments  are  imperishable  but  his 
theory  has  pa  sed  away.  For  a  century  it 
stood  like  a  dam  across  the  course  of  discov- 
ery ;  but,  like  all  barriers  that  rest  upon 
authority,  and  no.  upon  truth,  the  pressure 
from  behind  increased,  and  eventually  swept 
the  barrier  away.  This,  as  you  know,  was 
done  mainly  through  the  labors  of  Thomas 
Young,  and  his  illustrious  French  fellow- 
worker  Fresnel. 

In  1808,  Malus,  looking  through  Iceland 
spar  at  the  sun  reflected  from  the  window  of 
the  Luxembourg  Pa!ace  in  Paris,  discovered 
the  polarization  of  light  by  reflection.  In 
18 1 1  Arago  discovered  the  splendid  chro- 
matic phenomena  which  we  have  had  illus- 
trated by  plates  of  gvp-um  i:i  polarized  lignt ; 
he  also  discovered  the  rotation  of  the  plane 
of  polarization  by  quartz-crystals.  In  1813 
Seebeck  discoveied  the  polaiiz  tion  of  light 
by  tourmaline.  That  same  y^ar  Brews. er 
discovered  those  magnificent  ban,  s  of  co.or 
that  surround  the  axes  of  biaxal  cry  ta's.  In 
1814  Wollaston  discovered  the  ri.i^s  of  Ice- 
land spar.  All  these  erkcU,  \wiuh,  without 
a  theoretic  clue,  would  leave  the  human 
mind  in  a  hopeless  jungle  of  phenonena 
without  harmony  or  relation,  were  organically 
connected  by  the  theory  of  undulation.  The 
theory  was  applied  and  verified  in  all  direc- 
tions, Airy  being  especially  conspicuous  for 
the  severity  and  conclusiveness  of  his  proofs. 
The  most  remarkable  ver.fication  fell  to  the 
lot  of  the  late  Sir  William  Hamilton,  of  Dub- 
lin, a  profound  mathematician,  who.  taking 
up  the  theory  where  Fresnel  had  left  it,  ar- 
rived at  the  conclusion  that,  at  fo  ir  special 
points  at  the  surface  of  the  ether-wave  in 
double-refracting  crystals,  lie  ray  was  divided 
not  into  two  parts,  but  into  an  infinite  num- 
ber of  parts  ;  forming  at  these  points  a  con- 
tinuous conical  envelope  instead  of  two 
images.  No  human  eye  had  ever  seen  this 
envelope  when  Sir  WillLm  Hamilton  inferred 
its  existence.  Turning  to  nis  fri.nJ  Dr. 
Lloyd,  he  asked  him  to  test  experimentally 
the  truth  of  his  theoretic  conclusion.  Lloyd, 
taking  a  crystal  of  arragonite,  and  following 
with  the  most  scrupulous  exactness  the  indi- 
cations of  theory,  cutting  the  crystal  wheie 
theory  said  it  ought  to  be  cut,  observing  it 
.here  theory  said  it  ought  to  be  observed, 
found  the  luminous  envelope  which  had  pre 
viousiy  been  a  mere  idea  in  the  mind  of  the 
malhem.ttici  in. 

.\e\ertneless  this  great    theory   cf    undula- 
tion, like    many   another  truth,  which   in  the 
m  has  proved  a  blessing  t>    humanity. 


SIX  LECTURES   ON  LIGHT. 


had  to  establish,  by  hot  conflict,  its  right  to 
exist*  nee.  Great  names  were  arrayed  against 
it.  It  had  been  enunciated  by  Hooke,  it  had 
been  applied  by  Huyghens,  it  had  been  de- 
fended by  Euler.  But  they  made  no  impres- 
sion. And,  indeed,  the  theory  in  their  hands 
was  more  an  analogy  than  a  demonstration. 
It  first  took  the  form  of  n  demonstrated  verity 
in  the  hand  of  Tho.nas  Young.  He  broug  it 
the  waves  of  light  to  bear  upon  each  other, 
causing  them  to  support  each  other,  and  to 
extinguish  each  ether  at  will.  From  their 
mutual  actions  he  determined  their  lengths, 
and  applied  his  determinations  in  all  direc- 
tions. He  showed  t  at  the  standing  difficulty 
of  polarization  might  be  embraced  by  the 
theory.  After  him  came  Fresnel,  whose 
transcendent  mathematical  abilities  enat  led 
him  to  give  the  theory  a  generality  unattained 
by  Young.  He  grasped  the  theory  in  its 
entirety  ,  followed  the  ether  into  its  eddies 
and  estuaries  in  the  hearts  of  crystals  of  the 
mo>t  complicated  structure,  and  into  bodies 
subjected  to  strain  and  pressure.  He  showed 
that  the  facts  discovered  by  Malus,  Arago, 
Brewster,  and  Biot,  were  so  many  ganglia, 
so  to  speak,  of  his  theoretic  organism,  deriv 
ing  from  it  sustenance  and  explanation. 
With  a  mind  too  strong  for  the  body  with 
which  it  was  associated,  that  body  became  a 
wreck  long  before  it  had  become  old,  and 
Fresnel  died,  leaving,  however,  behind  him  a 
name  immortal  in  the  annals  of  science. 

One  word  more  I  should  like  to  say  regard- 
ing Fresnel.  There  are  things,  ladies  and 
gentlemen,  better  even  than  science.  There 
are  matters  of  the  character  as  well  as  matters 
of  thj  intellect,  and  it  is  always  a  pleasure  to 
those  who  wish  to  think  well  01  human  na- 
ture, when  high  intellect  and  upright  charac- 
ter are  combined.  They  were,  I  believe, 
combined  in  this  young  Frenchman.  In 
thos:  hot  conflicts  of  the  undulatory  theory, 
he  stood  forth  as  a  man  of  integrity,  claiming 
no  more  than  his  right,  and  ready  to  concede 
their  rights  to  others.  He  at  once  recognized 
and  acknowledged  the  merits  of  Thomas 
Young.  Indeed,  it  was  he,  and  his  fellow, 
countryman  Arago,  who  first  startled  England 
into  the  consciousness  of  the  injustice  done 
to  Young  in  the  Edinburgh  Review.  1  should 
like  to  read  you  a  brief  extract  from  a  letter 
written  by  Fresnel  to  Young  in  1824,  as  it 
ihrows  a  pleasant  light  upon  the  character  of 
the  Frenc  \  philosopher.  '*  For  a  long  time," 
says  Fresnel,  "that  sensibility,  or  that 
vanity,  which  people  call  love  of  glory,  has 
been  much  blunted  in  me.  I  labor  much 
less  to  catch  the  suffrages  of  the  public  than 
to  obtain  that  inward  approv  :i  which  has 
always  been  the  sweetest  rewarxi  of  my  efforts. 
Without  doubt,  in  moments  of  disgust  and 
discouragement,  I  have  often  needed  the  spur 
(  f  vanity  to  excite  me  to  pursue  my  researches. 
But  all  the  compliments  I  have  received  from 
Arago,  De  la  Place,  and  Biot,  never  gave  me 
so  much  pleasure  as  the  discovery  of  a.  theo- 


retic truth,  or  the  confirmation  of  a  calcula- 
tion by  experiment." 

This,  ladies  and  gentlemen,  is  the  core  of 
the  whole  matter  as  regard^  science.  It 
must  be  cultivated  for  its  own  sake,  for  the 
pure  love  of  truth,  rather  than  for  the  ap- 
plause or  profit  that  it  brings.  And  now  my 
occupation  in  America  is  wellnigh  g~ne. 
Still  I  will  bespeak  your  tolerance  for  a  few 
concluding  remarks  in  reference  to  the  men 
who  have  bequeathed  to  us  the  vast  body  of 
knowledge  of  which  I  have  sought  to  give 
you  some  faint  idea  in  these  lectures.  What 
was  the  motive  that  spurred  them  on?  what 
the  prize  of  their  high  calling  for  whici  they 
struggled  so  assiduously?  What  urged  them 
to  those  battles  and  those  victories  over  reti- 
cent Nature  which  have  become  the  heritage 
of  the  human  race  ?  It  is  never  to  be  for- 
gotten  that  not  one  of  those  great  investiga- 
tors, from  Aristotle  down  to  Stokes  and 
Xirchhoff,  had  any  practical  end  in  view, 
according  to  the  ordinary  definition  of  the 
word  "practical."  They  di  1  not  propose  to 
themselves  money  as  an  end,  and  knowledge 
as  a  means  of  obtaining  it.  For  the  most 
part,  they  nobly  reversed  this  process,  made 
knowledge  their  end,  and  such  money  as 
they,  possessed  the  means  of  obtaining  it. 

We  may  see  to-day  the  issues  of  their 
work  in  a  thousand  practical  forms,  and  this 
may  be  thought  sufficient  to  justify  it,  if  not 
ennoble  their  efforts.  But  they  did  not  work 
for  such  issues  >  their  reward  wa  ,of  a  totally 
different  kind.  We  love  clothes,  we  love 
luxuries,  we  love  fine  equipages,  we  lovt 
money,  and  any  man  who  can  point  to  these 
as  the  result  of  Us  efforts  in  lite  justifies 
these  efforts  before  all  the  world.  In  Ameri- 

a 

con- 

fidently to  this  assembly  whether  such  thing* 
exhaust  the  demands  of  human  nature?  Tht 
very  presence  here  for  six  inclen  ent  nights 
of  this  audience,  embodying  so  much  Oi  the 
mental  force  and  refinement  of  this  great 
city,  is  an  answer  to  my  question  I  neetf 
not  tell  such  an  assembly  that  there  are  joy; 
of  the  intellect  as  well  as  joys  of  the  body, 
or  that  these  pleasures  of  the  spirit  constu 
tuted  the  reward  of  our  great  investigators. 
Led  on  by  the  whisperings  of  natural  truth, 
through  pain  and  self-denial,  they  often  pur- 
sued their  work.  With  the  ruling  passion 
strong  in  death,  some  of  tnem,  when  no 
longer  able  to  hold  a  pen,  dictated  to  their 
friends  the  result  of  their  labors,  and,  then 
rested  from  them  forever. 

Could  we  have  seen  these  men  at  work 
without  any  knowledge  of  the  consequences 
of  their  work,  what  should  we  have  thought 
of  then\?  To  many  of  their  contemporaries 
it  would  have  appeared  simply  ridiculous  to 
see  men,  whose  namei  are  now  stars  in  the 
firmanent  of  science,  straining  their  atten- 
tion to  observe  an  effect  of  experiment  al- 
most too  minute  for  detection,  To  the  un- 


. 

ca   and   England    more   especially   he    is 
"  practical  "  man.     But  I  would  appeal  co 


SIX  LECTURES  ON  LIGHT. 


initiated,  they  might  well  appear  as  big 
children  playing  with  not  very  amusing  toys. 
It  is  so  to  this  hour.  Could  you  watch  the 
true  investigator — your  Henry  or  your  Dra- 
uer,  for  example — in  his  laboratory,  unless 
animated  by  his  spirit,  you  could  hardly  un- 
derstand what  keeps  him  there.  Many  of 
the  objects  which  rivet  his  attention  might 
jppear  to  you  utterly  tiivial  ;  and,  if  you  were 
cO  step  forward  and  ask  him  what  is  the  use  of 
his  work,  the  chances  are  that  you  would 
confound  him.  He  might  not  be  able  to 
express  the  use  of  it  in  intelligible  terms. 
He  might  not  be  able  to  assure  you  that  it 
will  put  a  dollar  into  the  pocket  of  any 
human  being  living  or  to  come.  That 
scientific  discovery  may  put  not  only  dollars 
into  the  pockets  of  individuals,  but  millions 
into  the  exchequers  ot  nations,  the  history  of 
science  amply  proves  ;  but  the  hope  of  its 
doing  so  never  was  and  never  can  be  the 
motive  power  of  the  investigator. 

I  know  that  1  run  some  risk  in  speaking 
thus  before  practical  men.  I  knojv  what  De 
Tocqueville  says  of  you.  "  The  man  of  the 
North,"  he  says,  "has  not  only  experience, 
but  knowledge.  He,  however,  does  not  care 
for  science  as  a  pleasure,  and  only  embraces 
t  with  avidity  when  it  leads  to  useful  appli- 
cations." But  what,  I  would  ask,  are  the 
hopes  of  useful  applications  which  have 
drawn  you  so  many  times  to  this  place  in 
>pite  of  snow-drifts  and  biting  cold  ?  What, 
I  may  ask,  is  the  origin  of  that  kindness 
which  drew  me  from  my  work  in  London  to 
address  you  here,  and  which,  if  I  permitted 
it,  would  send  me  home  a  millionaire  ?  Not 
because  I  had  taught  yot*  to  make  a  single 
cent  by  science,  am  I  among  you  to-night, 
but  because  I  tried  to  the  best  of  my  ability 
iO  present  science  to  the  world  ss  an  intellec- 
tual good.  Surely  no  two  terms  were  ever 
so  distorted  and  misapplied  with  refe.ence  to 
man  in  his  higher  relations  as  these  terms 
useful  and  practical.  As  if  there  were  no 
nakedness  of  the  mind  to  be  clothed  as  well 
as  nakedness  of  the  body — no  hunger  and 
thirst  of  the  intellect  to  s.itisfy.  Let  us  ex- 
pand the  definitions  of  these  terms  until  they 
embrace  all  the  needs  of  man,  his  highest  in- 
tellectual needs  inclusive.  It  is  specially  on 
this  ground  of  its  administering  to  the  higher 
needs  of  intellect,  it  is  mainly  because  I  be- 
lieve it  to  be  wholesome  as  a  source  of  know- 


tion.  You  are  delighted,  and  with  good 
reason,  with  your  electric  telegrrphs,  proud 
of  your  steam-engines  and  your  factories, 
and  charmed  with  the  productions  of  pho- 
tography. You  see  daily,  with  just  elation, 
the  creation  of  new  forms  of  industry  — 
new  powers  of  adding  to  the  wealth  and 
comfort  of  society.  Industrial  England  is 
heaving  with  forcts  tending  to  this  end,  and 
the  pulse  of  industry  beats  still  stronger  in 
the  United  States.  And  yet,  when  analyzed, 
what  are  industrial  America  and  industrial 
England?  If  you  can  tolerate  freedom  of 
peech  on  my  part,  I  will  answer  this  ques- 
tion by  an  illustration.  Stiip  a  strong  arm, 
and  regard  the  knotted  muscles  when  the 
hand  is  clenched  and  the  arm  bent.  Is  this 
exhibition  of  energy  the  work  of  the  muscle 
alone?  By  no  means.  The  muscle  is  the 
channel  of  an  influence,  without  which  it 
would  be  as  powerless  as  a  lump  of  plastic 
dough.  It  is  the  delicate  unseen  nerve  that 
unlocks  the  power  of  the  muscle.  And, 
without  those  filaments  of  genius  which  have 
been  shot  like  nerves  through  the  body  of 
society  by  the  original  discoverer,  industrial 
America  and  industrial  England  would,  I 
fear,  be  very  much  in  the  condition  of  that 
plastic  dough. 

At  the  present  time  there  is  a  cry  in  Eng- 
land for  technical  education,  and  it  is  the  ex- 
pression of  a  true  national  want  ;  but  there 
is  no  cry  for  original  investigation.  Still 
without  this,  as  surely  as  the  stream  dwindles 
when  the  spring  dries,  so  surely  will  "tech- 
nical education"  lose  all  foice  of  growth,  all 
power  of  reproduction.  Our  great  investi- 
gators have  given  us  sufficient  work  for  a 
time  ,  but,  if  their  spirit  die  out,  we  shall 
find  ourselves  eventually  in  the  condition  of 
those  Chinese  mentioned  by  De  Tocqutville. 
who,  having  forgotten  the  scientific  origin  of 
what  they  did,  were  at  length  compelled  to 
copy  without  variation  the  inventions  of  an 
ancestry  who,  wiser  than  themselves,  had 
drawn  their  inspiration  direct  from  Nature. 

To  keep  society  as  regards  science  in 
healthy  play,  three  classes  of  workers  are 
necessary:  Firstly,  the  investigator  of  natura/ 
truth,  whose  vocation  it  is  to  pursue  that 
truth,  and  exteud  the  field  of  discovery  for 
the  truth's  own  sake,  and  without  reference 
to  practical  ends.  Secondly,  the  teacher  of 
natural  truth,  whose  vocation  it  is  to  give 


ledge,   and  as  a  means  of  discipline,  that  I  I  public  diffusion  to  the  knowledge  already  won 
urge  the  claims  of  science  this  evening  upon  [  by  the   discoverer.     Thirdly,    the   applier  of 


your  attention. 

But.  with  refen  nee  to  material  needs  and 
joys,  surely  pure  science  has  also  a  word  to 
say.  People  sometimes  speak  as  if  steam  had 
not  been  studied  before  James  Watt,  or  elec- 
tricity before  Wheatslone  and  Morse  ;  where- 
as, in  point  of  fact,  Watt  and  Wheatstone 
and  Morse,  with  all  their  practicality,  were 
the  mere  outcome  of  antecedent  forces,  which 
acted  without  reference  to  practical  ends. 
This  also,  1  think,  me  its  u  moment's  atteu- 


natural  truth,  whose  vocation  it  is  to  make 
scientific  knowledge  available  for  the  needs, 
comforts,  and  luxuries  of  life.  These  three 
classes  ought  to  coexist  and  interact  Now, 
the  popular  notion  of  science,  both  in  this 
country  and  in  England,  often  relates,  not  to 
science  strictly  so  called,  but  to  .hj  appli  a 
tions  of  science.  Such  applic  uions.  esj.i- 
cially  on  this  continent,  are  so  as'ounding — 
they  spread  themselves  so  largely  and  um- 
brageously  before  the  public  e-ye — as  to  shut 


SIX  LECTURES  ON  LIGHT. 


out  from  view  those  workers  who  are  engaged 
in  thj  quieter  ami  profounder  business  of 
original  mvestiga'ion. 

Take  the  electric  telegraph  as  an  example, 
whi  h  has  been  repeatedly  forced  upon  my 
attention  of  late.  I  am  not  here  to  attenuate 
in  the  slightest  degree  the  services  of  those 
who,  in  England  and  America,  have  given 
the  telegraph  a  form  so  wonderfully  fitted  for 
public  use.  They  earned  a  great  reward, and 
assuredly  they  have  received  it.  But  I  should 
be  untrue  to  you  and  to  myself  if  I  failed  lo 
tell  you  that,  however  high  in  particular  re- 
spects their  claims  and  qualities  may  be,  prac- 
tical men  did  not  discover  the  electric  tele- 
grjph.  Th-i  discovery  of  the  electric  telegraph 
impiie  .  the  discovery  of  electricity  itself,  and 
the  development  of  its  laws  and  phenomena. 
Such  discoveries  are  not  made  by  practical 
men,  and  they  never  will  be  m  .do  by  them, 
because  their  minds  are  bes<_'t  with  ideas 
which,  though  of  the  highest  value  from  one 
point  of  view,  are  not  those  which  slimulate 
the  original  discoverer. 

The  ancients  discovered  the  electricity  of 
arnber;  and  Gilbert,  in  the  year  1600,  ex- 
tended the  force  to  other  bodies  Then  fol- 
lowed other  inquirers,  your  own  Franklin 
among  the  number.  But  this  form  of  elec- 
tricity, though  tried,  did  not  come  into  use 
lor  telegraphic  purposes.  Then  appeared  the 
great  Italian,  Volto,  who  discovered  the 
source  of  electricity,  which  bears  his  name, 
and  applied  the  most  profounu  Lisight  and 
the  most  delicate  experimental  skill  to  its  de- 
velopment. Then  arose  the  man  who  added 
to  the  powers  of  his  intellect  all  the  graces  of 
the  human  heart,  Michael  Faraday,  the  dis- 
coverer of  the  great  domain  of  magneto- 
electricity.  CErsted  discovered  the  deflection 
ol  the  magnetic  needle,  and  Arajpp  and  Stur- 
£eo  i  the  magnetization  of  iron  by  the  elec- 
tric current.  The  voltaic  circuit  finally  found 
*ts  theoretic  Newton  i  i  Ohm,  while  Henry, 
of  Princeton,  who  had  the  sagacity  to  recog- 


our  peril.  For  I  say  again,  that,  behind  afi 
your  practical  applications,  there  is  a  region 
of  intellectual  action  to  which  practical  men 
have  rarely  contributed,  but  from  which  they 
draw  all  their  supplies.  Cut  them  off  from 
this  region,  and  they  become  eventually  help- 
less. In  no  case  is  the  adage  truer,  "Other 
men  labored,  but  ye  are  entered,  in  o  their 
labors,"  than  in  the  case,  of,. the  discoverer 
and  the  applier  of  natural  truth.  But  now  a 
word  on  the  other  side.  While  I  say  that 
practical  men  are  not  the  men  to  make  the 
necessary  antecedent  <  iscoveries,  the  cases 
are  rare  in  which  the  discoverer  knows  how 
lo  turn  his  labors  tc  practical  account.  Dif- 
ferent qualities  of  mind  and  different  habits 
of  thought  are  needed  in  .he  two  cases;  and. 
while  I  wish  to  give  emphatic  utterance  to 
the  claims  of  those  whose  claims,  owing  to 
the  simple  fact  of  their  intellectual  elevation, 
ar:  often  misunderstood,  I  am  not  here  to 
exalt  the  one  class  of  workers  at  the  expense 
of  the  other.  They  are  the  necessary  supple- 
ments of  each  other;  but  remember  that  one 
class  is  sure  to  be  taken  care  of.  Ail  the  ma- 
terial rewards  of  society  are  already  within 
their  reac.i;  but  it  is  at  our  peril  th^twe  neg- 
lect to  provide  opportunity  for  those  studies 
and  pursuits  which  have  no  such  rewards, 
and  from  wnich,  therefore,  the  rising  genius 
of  the  country  is  incessantly  tempted  away. 

Pasteur,  one  of  the  most  eminent  member? 
of  the  Institute  of  Fiance,  in  accounting  for 
the  disastrous  overthrow  of  his  country  ana 
the  predominance  of  Germany  in  the  late 
war,  expresses  himself  thus:  "  Few  persons 
comprehend  the  real  origin  of  the  marvels  e»i 
industry  and  the  wealth  of  nations.  I  need 
no  further  proof  of  this  than  the  employment, 
more  and  more  frequent  in  official  language, 
and  in  writing  of  all  sorts,  of  the  erroneous 
expression  applied  science.  The  abandon- 
ment of  scientific  c  rcers  by  men  capable  of 
pursuing  them  with  distinction, was  recent!*- 
complaint d  of.  in  the  presence  of  a  minister 


uize  the  merits  of  Ohm  while  they  were  still  I  of   the  greatest   talent.     This  statesman  en- 


decried  in  his  own  country,  was  at  this  time 
;.n  the  van  of  experimental  inquiry. 

In  the  works  of  these  men  you  have  all 
rhe  materials  employed  at  this  hour  in  all  the 
forms  of  the  electric  telegraph.  Nay,  more  ; 
Gauss,  the  celebrated  astronomer,  and  Weber, 
the  celebrated  natural  philosopher,  both  pn> 
fessors  in  the  University  of  Gottingen,  wish- 
ing to  establish  a  rapid  mode  of  communication 
between  the  observatory  and  the  physical 
cabinet  of  the  university,  did  this  by  means 
of  an  electric  .  telegraph.  The  force,  in 
shot  t, 'had  been  discovered,  it  • laws  invest^ 
gated  and  made  sure,  the  most  complete 
mastery  of  i's  phenomena  had  been  attained, 
nay,  its  applicability  to  teleg.aphic  purposes 
demonstrated,  by  men  whose  sole  reward  for 
their  labors  was  the  noble  joy  of  discovery, 
and  before  your  practical  men  appeared  at 
all  upon  the  scene. 

Are  \v\;  to    ignore   all    this?     We  do  so  at 


deavored  to  show  that  we  ought  not  to  be 
surprised  at  this  result,  because  in  our  day 
the  reign  of  theoretic  science  yielded  place  to 
that  of  applied  science.  Nothing  coaid  be 
more  erroneous  than  this  opinion,  nothing,  I 
venture  to  say,  more  dangerous,  even  to 
practical  life,  than  the  consequences  which 
might  How  from  these  words.  They  have 
rested  on  my  mind  as  a  proof  of  the  imperi- 
ous necessity  of  reform  in  our  superior  edu 
Cation  There  exists  no  category  of  the 
sciences  to  which  the  name  of  applied  science 
could  be  given.  llre  have  science  ^  and  the 
applications  of  science  which  are  united  to- 
gether as  the  tree  and  its  (ruit." 

And  Cuvier,  the  great  comparative,  anato- 
mist, writes  thus  upon  the  same  theme' 
"  These  grand  practical  innovations  are  the 
mere  applications  of  truths  of  a  higher  order, 
not  sought  with  a  practical  intent,  but  which 
were  pursued  for  their  own  sake,  and  solely 


LECTURES  ON  LIGHT. 


through  an  ardor  for  knowledge.  Those 
who  applied  them  could  not  have  discoverec 
them;  those  who  discovered  them  had  no  in- 
clination to  pursue  them  to  a  practica.  end. 
Engaged  in  the  high  regions  whither  their 
thoughts  had  carried  them,  they  hardly  per- 
ceived these  practical  issues,  though  born  of 
their  own  deeds.  These  rising  workshops, 
these  peopled  colonies,  those  ships  which  fur- 
row the  seas — this  abundance,  this  luxury, 
this  tumult — all  this  comes  from  discoverers 
in  science,  and  it  all  remains  strange  to 
them.  At  the  point  where  science  merges 
into  practice,  they  abandon  it;  it  concerns 
them  no  more." 

When  the  Pilgrim  Fathers  landed  at  Ply- 
mouth Rock,  and  when  Penn  made  his  treaty 
with  the  Indians,  the  new-comers  had  to 
bui'.d  their  houses,  to  chasten  the  earth  into 
cultivation,  and  to  take  care  of  their  souls. 
In  such  a  community,  science,  in  its  more 
abstract  forms,  was  not  to  be  thought  cf. 
And,  at  the  present  hour,  when  your  hardy 
Western  pioneers  stand  face  to  face  with 
stubborn  Nature,  piercing  the  mountains 
and  subduing  the  forest  and  the  prairie,  the 
pursuit  of  science,  for  its  own  sake,  is  not  to 
be  expected.  The  first  need  of  man  is  food 
and  shelter  ;  but  a  vast  portion  of  this  con- 
tinent is  already  raised  far  beyond  this  need. 
The  gentlemen  of  New  York,  Brooklyn,  Bos- 
ton, Philadelphia,  Baltimore,  and  Washing- 
ton, have  already  built  their  houses,  and  very 
beautiful  they  are  ;  they  have  also  secured 
their  dinners,  to  the  excellence  of  which  I 
can  also  bear  testimony.  They  have,  in 
fact,  reached  that  precise  condition  of  well-- 
being and  independence  when  a  culture,  as 


words  repeated,  and,  if  necessary,  laid  to. 
heart.  In  a  work  published  in  1850,  he  says: 
"  It  must  be  confessed  that,  among  the  civil- 
ized peoples  of  our  ^ge,  there  are  few  in 
which  the  highest  sciences  have  made  so^ 
little  progress  as  in  the  United  States. "* 
He  declares  his  conviction  that,  had  you  been 
alone  in  the  universe,  you  would  speedily 
have  discovered  that  you  cannot  long  make: 
progress  in  practical  science,  without  culti- 
vating theoretic  science  at  the  same  time. 
But,  according  to  De  Tocqueville,  you  are- 
not  thus  alone.  He  refuses  to  separate 
America  from  its  ancestral  home  ;  and  it  is 
here,  he  contends,  that  you  collect  the  treas- 
ures of  the  intellect,  without  taking  the- 
trouble  to  create  them. 

De  Tocqueville  evidently  doubts  the  ca- 
pacity of  a  democracy  to  foster  genius  as  it 
was  fostered  in  the  ancient  aristocracies. 
41  The  future,"  he  says,  "will  prove  whether 
the  passion  for  profound  knowledge,  so  rare 
and  so  fruitful,  can  be  born  and  developed 
so  readily  in  democractic  societies  as  in  aris- 
tocracies. As  for  me,"  he  continues,  "I 
can  hardly  believe  it."  He  speaks  of  the  un- 
quiet feverishness  of  democratic  commu- 
nities, not  in  times  of  great  excitement,  for 
such  times  may  give  an  extraordinary  impe- 
tus to  ideas,  but  in  times  of  peace.  There 
s  then,  he  says,  "a  small  and  uncomfortable- 
agaitation,  a  sort  of  incessant  attrition  of 
man  against  man,  which  troubles  and  dis- 
tracts the  mind  without  imparting  to  it  either 
animation  or  elevation."  It  rests  with  you 
to  prove  whether  these  things  are  nece-sarily 
so — whether  the  highest  scientific  genius  can- 
not find  in  the  midst  of  you  a  tranquil  home. 


as  humanity  has  yet  reached,   may  be  I  I  should  be  loath  to  gainsay  so  keen  an  ob- 

'  server  and  so  profound  a  political  writer,  but, 


justly  demanded  at  their  hands.  They  have 
reached  that  maturity,  as  possessors  of  wealth 
and  leisure,  when  the  investigator  of  natural 
truth,  for  the  truth's  own  sake,  ought  to  find 
among  them  promoters  and  protectors. 


since  my  arrival  in  this  country,  I  have  been, 
unable  to  see  anything  in  the  constitution  of 
society  to  prevent  a  student  with  the  rcct  ot 
the  matter  in  him  from  bestowing  the  most.' 


Among  the  many  grave  problems  before  steadfast  devotion  on  pure  science.  If  great 
them  they  have  this  to  solve,  whether  a  re-  |  scientific  results  are  not  achieved  in  Ameri- 
public  is  able  to  foster  the  highest  forms  of  ca,  it  is  not  to  the  small  agitations  of  society 
genius.  You  are  familiar  with  the  writings 


that  I  should  be  disposed  to  ascribe  the  de- 

of  De  Tocqueville,  and  must  be  aware  of  the  ]  feet,  but  to  the  fact  that  the  men  among  you 
intense  sympathy  which  he  felt  for  your  insti-  |  who  possess  the  endo  vments  necessary  for 
tutions  ;.  and  this  sympathy  is  all  the  more  scientific  inquiry  are  laden  with  duties  of  ad- 

ministration    or   tuition   so   heavy   as  to   be 
utterly  incompatible  with  the  continuous  and 


valuable,  from  the  philosophic  candor  with 
which  he  points  out,  not  only  your  merits, 
but  your  defects  and  dangers.  Now,  if  I 
come  here  to  speak  of  science  in  America  in 
a  critical  and  captious  spirit,  an  invisible 
radiation  'from  my  words  and  manner  will 
enable  you  to  find  me  out,  and  will  guide 
your  treatment  of  me  to-night.  But,  if  I,  in 
no  unfriendly  spirit — in  a  spirit,  indeed,  th_ 
reverse  of  unfriendly — venture  to  repeat  be- 
fore you  what  this  great  historian  and  analyst 
of  democratic  institutions  said  of  America,  I 
am  persuaded  that  you  will  hear  me  out.  He 
wrote  some  thrre-and-tweniy  years  ago,  and 
perhaps  would  not  write  the  same  to-day  ; 


tranquil    meditation  which   original    investi- 
gation demands.       It    may   well    be 


whether  Henry  would  have  been  transformed 
into  an  administrator,  or  whether  Draper 
would  have  forsaken  science  to  write  history, 
if  the  original  investigator  had  been  honored 
as  he  ought  to  be  in  this  land  ?  I  hardly 
think  they  would.  Still  1  do  not  think  this 


*  II  faut  reconnaitre,  que  parmis  les  peuples  civil- 
ises de  nos  jours,  il  en  esc  pcu  chez  qui  les  hautea. 
sciences  aient  fait  moins  de  progres  qu'aux  Etats- 
Unis,  ou  qui  aient  fourni  moins  de  grands  artistes, 

jicriiaps   wouiu    1101    wruciiic    SKUIIC    lu-uay  ,  j  de  poetcs7UustreS,  et  de  celobrcs  ecrivains.     (De  U- 
but  it  will  do  nobody  any  harm  to  have  his  '  Democratic  en  Amenque,  etc.,  tom-ii.,  p.  36.) 


SIX  LECTURES  ON  LIGHT. 


state  of  things  likely  to  last.  In  America 
there  is  a  willingness  on  the  part  of  individu- 
als to  devote  their  fortunes,  in  *,he  matter  of 
education,  to  the  service  of  the  common- 
wealth, which  is  without  a  parallel  elsewhere; 
and  this  willingness  requires  but  wise  di- 
rection to  enable  you  effectually  to  wipe  away 
the  reproach  of  De  Tocquaville. 

Your  most  difficult  proWem  will  be  nof  to 
build  institutions,  but  to  make  men  ;  not  to 
form  the  body,  but  to  rind  the  spiritual  em- 
bers which  shall  kindle  within  that  body 
a  living  soul.  You  have  scientific  genius 
among  you ;  not  sown  broadcast,  believe 
me,  but  still  cattered  here  and  there.  Take 
a!l  unnecessary  impediments  out  of  its  way. 
Drawn  by  your  kindness  I  have  come  here  to 
give  these  lectures,  and,  now  that  my  visit  to 
America  has  become  almost  a  thing  of  the 
past,  I  look  back  upon  it  as  a  memory  with- 
out a  stain.  No  lecturer  was  ever  rewarded 


as  I  have  been.  From  this  vantage-ground, 
however,  let  me  remind  you  that  the  work  of 
the  lecturer  is  not  the  highest  work  ;  that  in 
science,  the  lecturer  is  usually  the!  distributor 
of  intellectual  wealth  amassed  by  better  men. 
It  is  not  solely,  or  even  chiefly,  as  lecturers, 
but  as  investigators,  that  your  men  of  genius 
ought  to  be  employed.  Keep  your  sympa- 
thetic eye  upon  the  originator  of  knowledge. 
|  Give  him  the  freedom  necessary  for  his  re- 
j  searches,  not  overloading  him.  either  with  the 
I  duties  of  tu  tion  or  of  administration,  not  de- 
manding from  him  so-called  practical  results 
— above  all  things,  avoiding  that  question 
which  ignorance  so  often  addresses  to  genius, 
"  What  is  the  use  of  your  work  ?  "  Let  him' 
make  truth  his  object,  however  unpractical 
for  the  time  being  that  truth  may*  appear. 
If  you  cast  your  bread  thus  upon  the  waters, 
then  be  assured  it  will  return  to  you,  thougb 
it  may  be  after  many  days. 


CONTENTS. 


JL0CT.     I.— INTRODUCTORY- .    .  2 

II.— Origin  of  Physical  Theories    ...  8 

III. — Relation  of  Theories  to  Experience  .  18 
IV. — Chromai;c  Phenomena  produced  by 

Crystals 27 


LECT.     V.— Range  of  Vision  and  Range  of  Ra- 
diation ..........     3^. 

VI. — Spectrum  Analysis.    ......    A, 


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No.  5.  Education,  Intellectual,  Moral,  and 
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Kingsley. 

No.    7.    The   Conservation    of   Energy, 

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No.    8.    The  Study  of  Languages,  brought 

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No.  9.  The  Data  of  Ethics.  By  Herbert 
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No.  15.  Longevity.  The  means  of  prolong- 
ing life  after  middle  age.  By  John 
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No.  ii.  } 

I 


No.  12. 


No.  13. 


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No.  21.  Th<>  Physical  Rasis  of  Life,  with 
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No.  23.  Scientific  SopMsms.  A  review  of 
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wright,  D.D. 

No.  24.    Ponular      Srlentific      Lectures, 

(illustrated).    By  Prof.  H.  Helmholtz. 

No.  25.  The  Origin  of  Nations.  ByProl. 
Geo.  Rawlinson,  Oxford  University. 

No.  26.    rJTlie  Evolutionist  at  Large.     By 

Grant  Allen. 

No.  27:    The    History   of    Landholding 

in  k%ii"land.     By  Joseph  Fisher, 
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No.  29.    Facts  and  Fictions  of  Zoology, 

(numerous  illustrations).     By  Andrew 
Wilson,  Ph.  D. 

No.  30.  The  Study  of  Words.  Parti.  By 
Richard  Chenevix  Trench. 

No.  31.    The  Study  of  Words.    Part  II. 

No.  32.  Hereditary  Traits  and  Other 
Essays.  By  Richard  A.  Proctor. 


THE  HUMBOLDT  LIBRARY  OF  SCIENCE. 


No.  33- 
No.  34. 

No.  35- 
No.  36. 
No.  37. 
No.  38. 

No.  39. 
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No.  41. 
No.  42. 
No.  43. 
No.  44- 
No.  45. 

NO.  46. 

No.  47. 
No.  48. 
No.  49. 


Vignettes  front  Nature.  By  Grant 
Allen. 

The    Philosophy    of    Style.      By 

Herbert  Spencer. 

Oriental  Religions.  By  John  Caird, 
Pres.  Univ.  Glasgow,  and  Others. 

Lectures  on  Evolution.  (Illus- 
trated}. By  Prof.  T.  H.  Huxley. 

Six  Lectures  on  Light.  (Illus- 
trated). By  Prof.  Tyndall. 

Geological  Sketches.  Parti.  By 
Archibald  Geikie,  F.R.S. 

Geological  Sketches.    Part  II. 

The  Evidence  of  Organic  Evo- 
lution. By  George  J.  Romanes, 
F.R.S. 

Current  Discussion  in  Science. 

By  W.  M.  Williams,  F.C.S. 

History  of  the  Science  of  Poli- 
tics. By  Frederick  Pollock. 

Darwin  and  Hnmboldt.  By  Prof. 
Huxley,  Prof.  Agassiz,  and  others. 

The  Dawn  of  Hi«tory.  Parti.  By 
G.  F.  Keary,  of  the  British  Museum. 

The  Dawn  of  History.    Part  II. 


The   Diseases   of  Memory. 

Th.    Ribot.      Translated     from 
French  by  J.  Fitzgerald,  M.A. 

The  Childhood  of  Religion. 

Edward  Clodd,  F.R.A.S. 


By 
the 


By 


Life  In  Nature.  (Illustrated).  By 
James  Hinton. 

The  Sun  :  its  Constitution,  its  Phe- 
nomena, its  Condition.  By  Judge 
Nathan  T.  Carr. 

No.  50.    money  and  the  Mechanism  of 

Exchange.     By  Prof.  W.  Stanley 
Jevons,  F.R.S.     Parti. 


No.  51. 
No.  52. 

No.  53. 
No.  54- 
No.  55- 

No.  56. 
No.  57- 
No.  58. 

No.  59. 
No.  60. 
No.  61. 


Money  and  the  Mechanism  of 
Exchange.    Part  II. 

Th«    Diseases    of  the    Will.    By 

Th.    Ribot.      Translated     from     the 
French  by  J.  Fitzgerald,  M.A. 

Animal    Automatism,  and   other 
Essays.  By  Prof.  T.H.Huxley,  F.R.S. 

The  Birth  and  Growth  of  Myth. 

By  Edward  Clodd,  F.R.A.S. 

The  Scientific  Basis  of  Morals, 

and  other  Essays.    By  William  King- 
don  Clifford,  F.R.S. 


Illusions. 
Illusions. 


By  James  Sully. 
Part  II. 


Part.  I. 


The  Origin  of  Species.    By  Charles 
Darwin.     Part  I.     (Double  number). 

The  Origin   of  Species.     Part  II. 
(Double  Number). 

The  Childhood   of  the  World. 

By  Edward  Clodd,  F.R.A.S. 

Miscellaneous  Essays.  By  Richard 
A.  Proctor. 


No.  62. 
No.  63. 
No.  64. 
No.  65. 

No.  66. 
No.  67. 

No.  68. 
No.  69. 

No.  70. 
No.  71. 

No.  72. 

No.  73- 


No.  74. 
No.  75. 
No.  76. 
No.  77- 


The  Religions  of  the  Ancient 
World.  By  Prof.  Geo.  Rawlinson, 
Univ.  of  Oxford.  (Double  number). 

Progressive  Morality.  By  Thomas 
Fowler,  LL-D  ,  President  of  Corpus 
Christi  Coll.,  Oxford. 

The  Distribution  of  Animal* 
and  Plants.  By  A.  Russell  Wal- 
lace and  W.  T.  Thistleton  Dyer. 

Conditions  of  Mental  Devel»p- 
ment«  and  other  Essays.  By  Wil- 
liam Kingdon  Clifford. 

Technical  Education,  and  other 
Essays  By  Thomas  H.  Huxley,  F:R.S. 

The  Black  Death.  An  account  of 
the  Great  Pestilence  of  the  i4th  Cen- 
tury. By  J.  F.  C.  Hecker,  M.D. 

Three  Essays.    By  Herbert  Spencer. 

Fetichism :  A  Contribution  to  An- 
thropology and  the  History  of  Reli- 
gion. By  Fritz  Schultze,  Ph.  D. 
(Double  number). 

Essays  Speculative  and  Practi- 
cal. By  Herbert  Spencer. 

Anthropology.  By  Daniel  Wilson, 
Ph.  D  With  Appendix  < 


ology. 


on  Archae- 
By  E.  B.  tylor,  F.R.S, 


The  Dancing  Mania  of  the  Ui  id- 
die  Ages.  By  J.  F.  C.  Hecker,  M.D. 

Evolution  in  History,  Lan- 
guage and  Science.  Four  ad- 
dresses delivered  at  the  London 
Crystal  Palace  School  of  Art,  Science 
and  Literature. 

The  Descent  of  Man,  and  Selec- 
tion in  Relation  to  Sex.  (Numerous 
Illustrations).  By  Charles  Darwin. 
Nos.  74,  75,  76  are  single  Nos.;  No.  77  is 
a  double  No. 


No.  78.  Historical  Sketch  of  the  Dis- 
tribution of  Land  in  Kug- 
land.  By  William  Lloyd  Birbeck, 
M.A. 

No.  79.  Scientific  Aspect  of  some  Famil- 
iar Things.  By  W.  M.  Williams. 

No.  80.  Charles  Darwin.  His  Life  and 
Work.  By  Grant  Allen.  (Double 
Number). 

No.  81.  The  Mystery  of  Matter,  and  the 
Philosophy  o  I"  Ignorance. 

Two  Essays  by  J.  Allanson  Pictou. 

No.  82.  Illusions  of  the  Senses,  and  other 
Essays.  By  Richard  A.  Proctor. 

No.  83.  Profit-Sharing  Between  Capi- 
tal and  Labor.  Six  Essays.  By 
Sedley  Taylor,  M  A. 

No.  84.    Studies    of    Animated    Nature. 

Four  Essays  on  Natural  History.    By 
W.  S.  Dallas,  F.L.S.,  and  Others. 

No.  85.  The  Essential  Nature  of  Relig- 
ion. By  J.  Allanson  Picton. 

No.  86.    The   Unseen     Universe,  and   the 

Philosophy  of  the  Pure  Sciences.    By 
Prof.  Wm.  Kingdon  Clifford,  F.R.S. 

No.  87.    The  Morphine  Habit.      By  Dr.  B. 

Ball,  of  the  Paris  Faculty  of  Medicine. 


THE  HUMBOLDT  LIBRARY  OF  SCIENCE. 


No.  88.  Science  and  Crlm«  and  other 
Essays.  By  Andrew  Wilson,  F.R.S.E. 

No.  89.  The  Genesis  of  Science.  By  Her- 
bert Spencer. 

No.  90.  Notes  on  Earthquake* :  with 
Fourteen  Miscellaneous  Essays.  By 
Richard  A.  Proctor. 

No  91.    The    Rise    of    Univer»i«les.      By 

S.  S.  I^aurie,  LL.D.    Double  number). 

No  92.  The  Formation  of  Vegetable 
Would  through  the  Action  of 
Earth  Worms.  By  Charles  Dar- 
win, LL.D.,  F.R.S  (Double  number). 

No.  93.    Scientific    Methods    *>f    Capllnl 

Punishment.    By  J.  Mount  Bleyer, 
M  D. 

No.  94.  The  Factors  of  Organic  Evolu- 
tion. By  Herbert  Spencer. 

No  95.    The  Diseases  of  Personality.   By 

Th.     Ribot       Translated    from    the 
French  by  J.  Fitzgerald,  M  A. 

No.  96.    A  Half-Century  of  rcience.    By 

Thomas  H.  Huxley,  and  Grant  Allen. 


No.  97. 


The    Pleasures   of  Life. 

John  Lubbock. 


By    Sir 


No.  98.  Cosmic  Emotion:  Also  the 
Teachings  of  Science.  By  Wil- 
liam Kingdon  Clifford. 

No.  99.    Nature    Studies.      By    Prof.  F.  R.   | 
Eaton     Lowe ;     Dr.    Robert    Brown, 
F  L  S  ;  Geo.  G.   Chisholm,    F  R  G  S.; 
and  James  Dallas,  F  L  S. 

No.  loo.    Science    and    Poetry,    with 

other  Essays.  By  Andrew  Wilson, 
F.R  S  E. 

No.  101.  ^Esthetics;  Dreams  and  Asso- 
ciation of  Ideas.  By  James 
Sully  and  Geo.  Croom  Robertson. 

No.  102.  Ultimate  Finance;  A  True 
Theory  of  Co-operatloii.  By 

William  Nelson  Black. 

No.  103.  The  Coming  Slavery  :  The  Sins 
of  Legislators;  The  Great 
Political  superstition.  By 

Herbert  Spencer. 

No.  104.  .Tropical  Africa.  By  Henry  Drum- 
mond,  F.R.S. 

No.  105.    Freedom     In     Science     a  n  «1 

Teaching.  By  Ernst  Haeckel,  of 
the  University  of  Jena  With  a  pref- 
atory Note  by  Prof.  Huxley. 

No.  106.    Force  and  Energy.     A  Theory 

of  Dynamics.    By  Grant  Allen. 

No.  107.  Ultimate  Finance.  A  True 
Theory  of  Wealth.  By  William 
Nelson  Black. 

No.  108.    English,    Past    and     Present. 

By  Richard  Chenevix  Trench.  Parti. 
(Double  number). 

No.  109.    English,    Past     and     Present. 

Part  II. 

No.  no.  The  Story  of  Creation.  A  Plain 
Account  of  Evolution.  By 

Edward  Clodd.     (Double  number). 
No.  in.    The  Pleasures   of  Life.    Part  II. 
By  Sir  John  Lubbock. 


No.  112.    Psychology    of   Attention.      By 

Th.  Ribot  Translated  from  the 
French  by  J  Fitzgerald,  M.A. 

No.  113.  Hypnotism.  Its  History  and  Devel- 
opment. By  Fredrik  Bjornstrom, 
M.D.,  Head  Physician  of  the  Stock- 
holm Hospital,  Professor  of  Psychia- 
try. Late  Royal  Swedish  Medical 
Councillor.  Authorized  Translation 
from  the  Second  Swedish  Edition  by 
Baron  Nils  Posse,  M.G  ,  Director  of 
the  Boston  School  of  Gymnastics. 
(Double  number). 

No.  114.    Christianity  and  Agnosticism. 

A  Controversy.  Consisting  of 
papers  contributed  to  The  Nineteenth 
Century  by  Henry  Wnoe,  D.D.,  Prcf. 
Thos.  'H.  Huxley,  The  Bishop  of 
Petersborouerh,  W.  II.  MMlock,  Mrs. 
Humphrey  Ward.  (Double  number). 

No  115.  Farwinism  :  An  Exposition  cf  the 
Theory  of  Natural  Selectic  n,  with 
some  of  its  Applications.  J'y  Alfred 
Russel  Wallace,  LL.D.,  F.L.S  ,  etc. 
Illustrated.  Parti.  (Double  number). 

No.  116.  l?ar\vi  ill  tin.  Illustrated.  Part  II. 
(Double  number). 


No.  117.    Modern    Science   and 

Thought.  By  S.  Laing.  Illustrated. 
(Double  number) 

No  118.  Modern  Science  and  modern 
Thought.  Part  II. 

No.  119.  The  Electric  Light  and  The  Stor- 
ing of  Electrical  Energy.  Illustrated. 
Gerald  Molloy,  D  D.,  D.Sc. 

No.  120.    The  Modern   Theory   of  Heat 

and  The  Sun  as  a  Storehouse  of 
Energy.  Illustrated.  Gerald  Molloy, 
D.D.,  D.Sc. 

No.  121.  Utilitarianism.  By  John  Stuart 
Mill. 

No.  122.  Upon  the  Origin  of  Alpine  and 
Italian  Lakes  and  upon  Glacial 
Erosion.  Maps  and  Illustrations.  By 
Ramsey,  Ball,  Murchison,  Studer, 
Favre,  Whymper  and  Spencer.  Part 
I.  (Double  number;. 


No.  123. 


Upon  the  Origin  of  Alpine  and 
Italian  Lakes,  Etc.,  Etc     Part  II. 


No.  124.    The    Quintessence     of    Social- 
ism.   By  Prof.  A.  Schaffle. 

(•Darwinism   and   Politics.     By 

David  G.  Ritchie,  M  A. 
Iso  I2M  Administrative    Mhillsm.     By 

I     Thomas  Huxley,  F.R.S. 

No.  126.    Phrsloarn^my  and  Expression. 

By  P  Mantegazza.  Illustrated.  Part 
I.  (Double  number). 

No.  127.    Physiognomy  and  Expression. 

Pa'rt  II      (Double  number). 

No.  128.    The     Industrial     Revolution. 

By  Arnold  Toynbee,  Tutor  of  Baliol 
College,  Oxford  With  a  short  mem- 
oir by  B  Jowett.  Part  I.  (Double 
number). 

No.  129.    The     Industrial      Revolution. 
Part  II.     (Double  number). 

No.  130.    The  Origin  of  the  Aryans.    By 

Dr  Isaac  Taylor  Illustrated  Part 
I.  (Double  number) 


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No.  131.  The  Origin  of  tlie  Aryans.  Part 
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No.  132     The  Evolution  of  Sex.    By  Prof. 

P  Geddes  and  J.   Arthur  Thomson. 

i  .vi  •     Illustrated.    Parti    (Double  number). 

No.  133.    The  Evolution  of  Sex.    Part  II. 

(Double  number) 

.No.  134.    The  Law  of  Private  Right.    By 
s,          George  H  ..Smith*-  (Double  number;. 

-No.  135.  Capital.  A  Critical  Analysis  of  Cap- 
italist Production.  By  Karl  Marx  : 
Part  I  ( Double  number) 

•No.  136.  Capital.  Part  il  (Double  number) 
'No.  137.  Capital.  Part  III.  (Double  number) 
'No.  138.  Capital.  Part  IV  (Double  number)  < 

No.  139.  L.i<£h  till  tig.  Thunder  and  Lightning 
Conductors.  Illustrated.  By  Gerald 
Molloy,  DD.,  DSc. 

1  No.  140;  What  is  Mu»ic?  With  an  appen- 
dix on  How  the  Geometrical  Lines 
have  their  Counterparts  in  Music 
By  Isaac  L.  Rice. 

No.  141.  Are  I  fie  Effects  of  Use  and  Dis- 
u»e  inherited?  By  William  Platt 
Ball 

No.  142  A  Vindication  of  th*  Rights  of 
Woman.  By  Mary  Wollstonecraft. 
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No.  145  Body  and  Mind.  By  WTilliam 
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No.  146  Social  I)i»eases  ami  Worse 
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FRS 

No.  147  The  Soiil  of  Man  under  Social- 
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Geological     Sketches    at     Home     and 
M> road.       By    Archibald    Geikie,    LL.D., 
F.R.S.,    Director-General  of  the  Geological 
Surveys  of  Great  Britain  and  Ireland. 
Cloth 75  cts 

Illusions:  A  Psychological  Study.     By 

James  Sully,  author  of  "Sensation  and  Intu- 
ition,'    "Pessimism,"   etc.     Cloth.    .   .  75  cts 

The  Pleasures  of  Life.  Part  I  and  Part 
II.  By  Sir  John  Lubbock,  Bart.  Two  Parts 
in  One.  Cloth 75  cts. 

English,  Past  and  Present.  Part  I.  and 
Part  II.  By  Richard  Chenevix  Trench,  D.D., 
Archbishop  of  Dublin.  Two  Parts  in  One. 
Cloth 75  cts 

Hypnotism:  Its  History  «nd  Present 
Development.  By  Fredrik  Bjornstrom, 
M  D.,  Head  Physician  of  the  Stockholm  Hos- 
pital, Professor  of  Psychiatry,  late  Royal 
Swedish  Medical  Councillor.  Cloth  .  .  75 "cts 


THE  HUMBOLDT  LIBRARY  OF  SCIENCE. 


The  Story  of  Creation.  A  Plain  Account 
of  Evolution.  By  Edward  Clodd,  F.R.A.S. 
With  over  eighty  illustrations 75  cts 

Christianity  and  Agnosticism.  A  con- 
troversy, consisting  of  papers  by  Henry 
Wace,  D.D.,  Prebendary  of  St.  Paul's  Cathe- 
dral; Principal  of  King's  College,  Condon. 
Professor  Thomas  H.  Huxley.— W.  C.  Magee, 
D.D.,  Bishop  of  Peterborough.— W.  H.  Mai- 
lock,  Mrs.  Humphrey  Ward.  Cloth  .  .  75  cts 

Darwinism :  An  Exposition  of  the 
Theory  of  Natural  Selection,  with 
some  of  its  applications.  By  Alfred  Russel 
Wallace,  LL.D.,  F.L.S.  With  portrait  of  the 
author,  colored  map,  and  numerous  illustra- 
tions. Cloth $1.25 

The  ablest  living  Darwinian  writer. — Cincin- 
nati Commercial  Gazette. 

The  most  important  contribution  to  the  study 
of  the  origin  of  species  and  the  evolution  of  man 
which  has  been  published  since  Darwin's  death. 
—New  York  Sun. 

There  is  no  better  book  than  this  in  which 
to  look  for  an  intelligent,  complete,  and  fair 
presentation  of  both  sides  of  the  discussion  on 
evolution. — New  York  Herald. 

.11  oder H  Science  and  Modern  Thought. 

A  Clear  and  Concise  View  of  the  Principal 
Results  of  Modern  Science,  and  of  the  Revo- 
lution which  they  have  effected  in  Modern 
Thought.  With  a  Supplemental  Chapter  on 
Gladstone's  "Dawn  of  Creation"  and 
"Proem  to  Genesis,"  and  on  Drummond 
"Natural  I,aw  in  the  Spiritual  World."  By 
S.  Laing.  Cloth 75  cts 

Upon  the  Origin  of  Alpine  and  Italian 
Lukes;  and  Upon  Glacial  Erosion. 

By  A.  C.  Ramsay,  F.R.S.,  Etc.;  John  Ball, 
M.R.I. A.,  F.L.S.,  Etc.;  Sir  Rodenck  I.  Mur- 
chison,  F.K.S.,  D.C.L.,  Etc.;  Prof.  B.  Studer, 
of  Berne ;  Prof.  A.  Favre,  of  Geneva  ;  and 
Edward  Whymper.  With  an  Introduction, 
and  Notes  upon  the  American  Lakes,  by  Prof. 
J.  W.  Spencer,  Ph.D.,  F  G.S  ,  State  Geologist 
of  Georgia.  Cloth 75  cts 

Physiognomy    and     Expression.      By 

Paolo  Mantegazza,  Senator ;  Director  of  the 
National    Museum   of    Anthropology,    Flor- 
ence ;    President    of   the  Italian  Society  of 
Anthropology.    With  Illustrations. 
Cloth fi.oo 

The  Industrial  Revolution  of  lite 
Eighteenth  Cemury  in  England. 

Popular  Addresses,  Notes,  and  other  Frag- 
ments. By  the  late  Arnold  Toynbee.  Tutor 
of  Balliol  College,  Oxford.  Together  with  a 
short  memoir  by  B.  Jowett,  Master  of  Balliol 
College,  Oxford.  Cloth $1.00 

The  Origin  of  the  Aryans.  An  Account  of 
the  Prehistoric  Ethnology  and  Civilization 
of  Europe.  By  Isaac  Taylor,  M.A.,  Litt.  D., 
Hon.  LL.D.  Illustrated.  Cloth  ....  $1.00 

The  Law  of  Private  Right.  By  George 
H.  Smith,  author  of  "Elements  of  Right, 
and  of  the  Law,"  and  of  Essays  on  "The 
Certainty  of  the  Law,  and  the  Uncertainty  of 
Judicial  Decisions,"  "The  True  Method  of 
Legal  Education,"  Etc.,  Etc.  Cloth  .  .  75  cts 


The  Evolution  of  Sex.  By  Prof.  Patrick 
Geddes  and  J.  Arthur  Thomson.  With  104 
Illustrations.  Cloth -.  .  $1.00 

Such  a  work  as  this,  written  by  Prof.  Geddes 
who  has  contributed  many  articles  on  the  same 
and  kindred  subjects  to  the  Encyclopaedia  Brit- 
tannica,  and  by  Mr.  J.  Arthur  Thomson,  is  not 
for  the  specialist,  though  the  specialist  may  find 
it  good  reading,  nor  for  the  reader  of  light  liter- 
ature, though  the  latter  would  do  well  to  grapple 
with  it.  Those  who  have  followed  Darwin, 
Wallace,  Huxley  and  Haeckel  in  their  various 
publications,  and  have  heard  of  the  late1"  argu- 
ments against  heredity  brought  forward  by  Prof. 
Weissman,  will  not  be  likely  to  put  it  down 
unread.  .  .  .  The  authors  have  some  extremely 
interesting  ideas  to  state,  particularly  with 
regard  to  the  great  questions  of  sex  and  environ- 
ment in  their  relation  to  the  growth  of  life  on 
earth.  .  .  .  They  are  to  be  congratulated  on  the 
scholarly  and  clear  way  in  which  they  have 
handled  a  difficult  and  delicate  subject.—  Time s. 

Capital :  A  Critical  Analysis  of  Capi- 
talistic Production.  By  Karl  Marx. 
Translated  from  the  third  German  edition 
by  Samuel  Moore  and  Edward  Aveling,  and 
edited  by  Frederick  Engels.  The  only  A  meri- 
can  Edition.  Carefully  Revised.  Cloth,  $1.75 

The  great  merit  of  Marx,  therefore,  lies  in 
the  work  he  has  done  as  a  scientific  inquirer  into 
the  economic  movement  of  modern  times,  as  the 
philosophic  historian  of  the  capitalistic  era.— 
Encyclopedia  Brittannica. 

So  great  a  position  has  not  been  won  by  any 
work  on  Economic  Science  since  the  appearance 
of  The  Wealth  of  Nations.  .  .  .  All  these  circum- 
stances invest,  therefore,  the  teachings  of  this 
particularly  acute  thinker  with  an  interest  such 
as  cannot  be  claimed  by  any  other  thinker  of  the 
present  day.—  The  Athenceum. 

"What  is  Property?  An  Inquiry  into  the 
Principle  of  Right  and  of  Government.  By 
P.  J.  Proudhon.  Cloth $2.00 

Tlid  Philosophy  of  Misery.  A  System  of 
Economical  Contradictions.  By  P.  J  Proud- 
hon. Cloth $2.00 


Works  by  Professor  Huxley. 

Evidence  as  to  Man's  Place  in  Nature. 

With  numerous  illustrations 

AND 

On  the  Origin  of  Species ;  or,  the  Causes 
of     the     Phenomena     of     Organic 
Nature. 
Two  books  in  one  volume.    Cloth  ...  75  cts 


The  Physical  Basis  of  L,ife.    With  other 
Essays 

AND 

Lectures  on  Evolution.    With  an  Appen- 
dix on  the  Study  of  Biology. 
Two  books  in  one  volume.    Cloth  ...  75  cts 


A  Remarkable  Book. — Edward  Bellamy. 

THE 

KINGDOM  OF  THE  UNSELFISH; 

OR, 
EMPIRE  OF  THE  WISE. 

BY  JOHN  LORD  PECK. 

Cloth,   I2mo $1.00. 

"  Should  be  re-read  by  every  seeker  after  truth." — Rockland  Independent. 
"  Polished  in  style  and  very  often  exquisite  in  expression." — Natick  Citizen. 

"The  book  is  interesting  throughout,  and  the  more  widely  it  is  read  the  better."— 
tion." — Twentieth  Century. 

"Shows  profound  research,  original  ideas,  and  what  might  be  almost  called  inspira- 
tion."— Sunday  Times  (Tacoma). 

"The  effort  is  noble,  and  the  author  has  not  escaped  saying  many  profound  and  true 
things." — Christian  Union. 

"  One  of  a  large  number  of  '  reformatory  '  volumes  now  being  printed,  but  it  is  better 
than  many  of  them." — Truth  Seeker. 

"  The  book  is  from  a  widely-read  man,  and  is  written  for  a  high  end.  In  its  intellectual 
and  '  spiritual '  aspects,  it  is  educative  and  stimulating." — The  New  Ideal. 

"  The  book  before  us  is  one  of  the  signs  of  the  times.  It  prophesies  a  new  age,  and 
exhorts  to  the  life  which  shall  further  its  coming." — New  Church  Messenger. 

"  The  book  is  a  natural  product  of  the  prophetic  element  of  the  times,  which  is  reach- 
ing forward  into  the  new  economic  age  we  are  just  entering." — Teachers  Outlook. 

"  The  chapters  on  '  Natural  and  Social  Selection '  are  among  the  most  interesting  in  the 
book,  and  require  close  reading  to  take  in  the  whole  drift  of  their  meaning." — Detroit 
Tribune. 

"  It  is  a  real  contribution  to  original  and  advanced  thought  upon  the  highest  themes  of 
life  and  religion — of  intellectual,  moral,  social,  material  and  spiritual  progress." — The 
Unitarian. 

"  There  are  many  golden  sentences  in  the  chapter  on  Love,  and  the  practical  good 
sense  shown  in  the  treatment  of  the  marriage  question  would  help  many  husbands  and 
wives  to  live  more  happily  together." — The  Dawn. 

"  This  a  new  and  thoroughly  original  treatment  of  the  subjects  of  morality,  religion 
and  human  perfectibility,  and  furnishes  a  new  ground  for  the  treatment  of  all  social  ques- 
tions. It  is  radical  and  unique." — The  Northwestern. 

"  It  is  in  no  sense  an  ordinary  work.  It  makes  strong  claims  and  attempts  to  carry  out 
the  largest  purposes.  Taking  the  standpoint  of  science,  it  attacks  the  gravest  problems  of 
the  times  with  an  endeavor  to  show  that  the  most  advanced  science  will  enable  us  to  reach 
the  most  satisfactory  conclusions." — Chicago  Inter-Ocean. 

"  One  of  the  most  important  recent  works  for  those  who  are  striving  to  rise  into  a 
nobler  life,  who  are  struggling  to  escape  the  thraldom  of  the  present  selfish  and  pessimistic 
age.  Many  passages  in  Mr.  Peck's  work  strongly  suggest  the  lofty  teachings  of  those 
noblest  of  the  ancient  philosophers,  the  Stoics.  Those  who  are  hungering  and  thirsting 
after  a  nobler  existence  will  find  much  inspiration  in  '  The  Kingdom  of  the  Unselfish."  "• 
The  Arena. 

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